This commit changes the contract for the combinator parse functions:

 (1) The input state on failure must retain valid overrun and last_chunk
     fields. The latter is never changed, but overrun would be cleared by
     various combinators that backtrack in case of failure. All other
     fields of the input stream are still considered indeterminate after a
     failed parse.

 (2) If an overrun condition is encountered before the final chunk
     (last_chunk is false), the parse *must* fail. A helper want_suspend()
     is introduced as a shorthand for this check.

Fixes the packrat/iterative/dummy test.

right now there is duplication as the enum backend value is still
present- need to find out if it's OK to make a breaking change for
anyone who is for reasons known only to themselves chekcing which
backend enum value is one a parser they may have created and compiled...

Original behavior:

    hammer/build/debug/src/bindings/python$ LD_LIBRARY_PATH=. gdb python
    ...
    (gdb) r
    ...
    Python 2.7.6 (default, Nov 12 2018, 20:00:40)
    [GCC 4.8.4] on linux2
    Type "help", "copyright", "credits" or "license" for more information.
    >>> if 1:
    ...     import hammer as h
    ...     h.choice(h.ch_range('0', '9'), h.ch_range('A', 'Z'), h.ch_range('a', 'z')).compile(h._PB_GLR)
    ...

    Program received signal SIGSEGV, Segmentation fault.
    0xb79ecab2 in collect_nts (grammar=0x836abe0, symbol=0xb7d550a4)
        at build/debug/src/cfgrammar.c:120
    120	      for(x = (*s)->items; *x != NULL; x++) {
    (gdb) bt
    #0  0xb79ecab2 in collect_nts (grammar=0x836abe0, symbol=0xb7d550a4)
        at build/debug/src/cfgrammar.c:120
    #1  0xb79ecacd in collect_nts (grammar=0x836abe0, symbol=0x836ab58)
        at build/debug/src/cfgrammar.c:121
    #2  0xb79ec90a in h_cfgrammar_ (mm__=0xb79ff4b4 <system_allocator>,
        desugared=0x836ab58) at build/debug/src/cfgrammar.c:66
    #3  0xb79e8207 in h_lalr_compile (mm__=0xb79ff4b4 <system_allocator>,
        parser=0x836ab40, params=0x0) at build/debug/src/backends/lalr.c:280
    #4  0xb79e634a in h_glr_compile (mm__=0xb79ff4b4 <system_allocator>,
        parser=0x836ab40, params=0x0) at build/debug/src/backends/glr.c:15
    #5  0xb79f0eef in h_compile__m (mm__=0xb79ff4b4 <system_allocator>,
        parser=0x836ab40, backend=PB_GLR, params=0x0)
        at build/debug/src/hammer.c:97
    #6  0xb79f0e9d in h_compile (parser=parser@entry=0x836ab40, backend=PB_GLR,
        params=params@entry=0x0) at build/debug/src/hammer.c:92
    #7  0xb7a54ca4 in HParser__compile (backend=<optimized out>, self=0x836ab40)
        at hammer_wrap.c:3567

New behavior:

    hammer/build/debug/src/bindings/python$ LD_LIBRARY_PATH=. gdb python
    ...
    (gdb) r
    ...
    Python 2.7.6 (default, Nov 12 2018, 20:00:40)
    [GCC 4.8.4] on linux2
    Type "help", "copyright", "credits" or "license" for more information.
    >>> if 1:
    ...     import hammer as h
    ...     h.choice(h.ch_range('0', '9'), h.ch_range('A', 'Z'), h.ch_range('a', 'z')).compile(h._PB_GLR)
    ...
    True
    >>> ^D
    [Inferior 1 (process 19621) exited normally]
    (gdb) quit

After thrashing about for a few hours, this was the crucial clue:

    >>> import hammer as h
    >>> h.choice(h.ch('0')).compile(h._PB_GLR)
    ==18856== Conditional jump or move depends on uninitialised value(s)
    ==18856==    at 0x4A34FE9: h_desugar (desugar.c:7)
    ==18856==    by 0x4A2D150: h_desugar_augmented (lalr.c:261)
    ==18856==    by 0x4A2D1F7: h_lalr_compile (lalr.c:280)
    ==18856==    by 0x4A2B349: h_glr_compile (glr.c:15)
    ==18856==    by 0x4A35EEE: h_compile__m (hammer.c:97)
    ==18856==    by 0x4A35E9C: h_compile (hammer.c:92)
    ==18856==    by 0x49B0CA3: HParser__compile (hammer_wrap.c:3567)

The particular thing that it's saying is uninitialized seems to be

      if(parser->desugared == NULL) {

The `parser` in question is the `HParser` we're trying to desugar,
which is presumably the choice object created by `h.choice`, which
seems to be invoking `h_choice__a`:

    def choice(*args): return _h_choice__a(list(args))

That was implemented as follows:

    HParser* h_choice__a(void *args[]) {
      return h_choice__ma(&system_allocator, args);
    }

    HParser* h_choice__ma(HAllocator* mm__, void *args[]) {
      size_t len = -1; // because do...while
      const HParser *arg;

      do {
        arg=((HParser **)args)[++len];
      } while(arg);

      HSequence *s = h_new(HSequence, 1);
      s->p_array = h_new(HParser *, len);

      for (size_t i = 0; i < len; i++) {
        s->p_array[i] = ((HParser **)args)[i];
      }

      s->len = len;
      HParser *ret = h_new(HParser, 1);
      ret->vtable = &choice_vt;
      ret->env = (void*)s;
      ret->backend = PB_MIN;
      return ret;
    }

Indeed it does not seem to have been initializing `desugared`.  Fixing
this cures this symptom.

Other things it's probably worth checking out:

- Are there other places where we create HParser objects where one or
  more fields may be uninitialized?
- Perhaps `h_new` should zero the memory it returns, since it's only
  used for fixed-size objects and not things like variable-size
  character buffers?

As MSVC doesn't implement C99, variable-length arrays
are not supported. We use _alloca instead.

h_sequence need PB_MIN set.

Conflicts:
	src/bindings/python/SConscript
	src/bindings/python/hammer_tests.py

h_sequence need PB_MIN set.

Conflicts:
	src/bindings/python/SConscript
	src/bindings/python/hammer_tests.py

Refactored all of the desugaring code to no longer depend on memory being initialized to 0. Everything is about 12% faster now.

Expanded vtable for methods to determine whether a combinator is ok to use with regular or CF grammars.