At some stage (in a few years) we will want to bump the MSRV again, this is a tracking issue for possible changes at such a time

• Use matches! macro - came in in Rust 1.42
• Use #[track_caller] in secp256k1 and maybe elsewhere; Rust 1.46
• slice patterns instead of copy_from_slice where relevant; Rust 1.42
• Remove unsigned_abs from src/util/amount and use i8::unsigned_abs(); Rust 1.51