Add `Fixed.list` combinator
Add a combinator for fixed-length list.
The main motivation behind this is to improve the implementation of the Compact.list
combinator from !70 (merged). Specifically, Compact.list
currently uses tuples to encode fixed-size lists. This is inefficient because of the amount of allocation and AST traversal during the encoding/decoding process.
Edited by Raphaël Proust