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/t_bitwriter.c

@@ -24,7 +24,7 @@ void run_bitwriter_test(bitwriter_test_elem data[], char flags) {
     .input = buf,
     .index = 0,
     .length = len,
-    .bit_offset = (flags & BIT_BIG_ENDIAN) ? 8 : 0,
+    .bit_offset = 0,
     .endianness = flags,
     .overrun = 0
   };