refactor: simplify some exact instances of Option/Result combinators

[huitseeker]

This replaces some pattern-matches that are exact reimplementation of stdlib combinators.

This, as well as running clippy on the last commit involved changes that assume a more recent version of Rust, e.g.:

• captured identifiers (Rust 1.58)
• boolean combinators (then, then_some, since Rust 1.62)

Opinions on this loosely held, feel free to suggest changes. First commit tool-aided by comby-rust

[huitseeker]

Cleaning my PR queue, feel free to re-open if interested.