Fix #118

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.

src/parsers/parser_internal.h

@@ -18,6 +18,7 @@ static inline HParseResult* make_result(HArena *arena, HParsedToken *tok) {
   HParseResult *ret = h_arena_malloc(arena, sizeof(HParseResult));
   ret->ast = tok;
   ret->arena = arena;
+  ret->bit_length = 0; // This way it gets overridden in h_do_parse
   return ret;
 }