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.
