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.

It's causing unreferenced-variable warnings, and isn't referenced
anywhere aside from benchmark.c. If client code is likely to reference
it, perhaps move it into another header, so people who include hammer.h
don't have to refer to it to have warning-free code.

Mention user guide in README

Scons now doesn't throw away PKG_CONFIG_PATH, so NixOS can build it

Scons now doesn't throw away /home/stewart/.nix-profile/lib/pkgconfig:/nix/var/nix/profiles/default/lib/pkgconfig:/run/current-system/sw/lib/pkgconfig, so NixOS can build it

add h_permutation