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.

Specify warning for internal only headers that are now installed for use
by extensions.

Refactored all of the desugaring code to no longer depend on memory being initialized to 0. Everything is about 12% faster now.