- defined: bool
- fast: [512] u16
- (length << 8) | symbol for every 9-bit prefix of a code of at most 9 bits, else 0.
- ac_fast: [512] u32
- For AC tables, per 9-bit prefix: the whole run/size code plus its extra bits when they fit
in 9 bits. Low byte: bits used. Next byte: run, or 255 for end of block. High half: the
coefficient. 0 when the prefix needs the general path.
- limit: [17] s32
- one past the last code of each length, left-aligned to 16 bits
- first_code: [17] s32
- first_index: [17] s32
- symbols: [256] u8