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.h

@@ -99,6 +99,7 @@ typedef struct HParsedToken_ {
   HTokenData token_data;
 #endif
   size_t index;
+  size_t bit_length;
   char bit_offset;
 } HParsedToken;