Add monadic bind combinator h_bind

Free res_unamb

Add some missing type conversions to make h_read_bits work with count>32

Descend properly in h_seq_index_path

Fix #118

H_VALIDATE_APPLY macro

NEWS:
* Switching endianness mid-byte no longer potentially re-reads bytes.
* bit_offset now consistently refers to the number of bits already
  read.
* HParsedTokens now have a bit_length field; this is a size_t.  This
  may be removed for memory reasons.

The bit writer has not yet been updated to match; the result of
switching bit writer endianness in the middle of a byte remains
undefined.

I've found this especially useful in combination with my own
_attr_uint_const for things like flags and type specifiers.

It's possible that its usefulness might be diminished significantly if
there were a built-in bitfield constant parser -- that certainly would
eliminate all of my current uses of it -- but it still seems nicely
symmetric with H_ACT_APPLY.

Tabs after the first non-tab character are crazymaking. I picked 8
because it wasn't 7 and caused the backslashes to line up on
H_ACT_APPLY.

llvm doesn't care about your size

Fix compilation when PKG_CONFIG_PATH is not in environ

Remove vacuous states in base64_sem?.c

Move backend names into src/benchmark.c

They apparently removed alloc_size at some point (it was a no-op
beforehand), causing the attribute to throw an error when clang compiles
anything including allocator.h.

I don't think these actually affect correctness since there's no way for
0x40 or 0x60 to show up in a parse tree anyway, but they're confusing.