Optimize trinucleotide with bitwise ops and Nuc layout change

Reordered `Nuc` enum variants and added `#[repr(u8)]` to enable efficient
bitwise operations in `to_int`. Replaced the large match statement in
`trinucleotide` with a direct calculation using this optimized lookup.

Performance:
- Reduces execution time by ~20% (1.73s -> 1.45s on example data).
- Reduces code size by ~500 lines.

Author: bmesuere