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/hammer.c

@@ -52,7 +52,7 @@ HParseResult* h_parse__m(HAllocator* mm__, const HParser* parser, const uint8_t*
   // Set up a parse state...
   HInputStream input_stream = {
     .index = 0,
-    .bit_offset = 8,
+    .bit_offset = 0,
     .overrun = 0,
     .endianness = BIT_BIG_ENDIAN | BYTE_BIG_ENDIAN,
     .length = length,