Merge pull request #2 from Crell/prep

Prep for release

Bump PHP requirement to 8.1.

Remove unnecessary else.

Split keyed and non-keyed versions of all() and any().

Add typeIs() utility.

PHPStan Level 8.

Handle uninitialized values in Evolvable.

Add head, tail, and headtail recursion functions.

Split out separate key-using versions of the filter functions, too.

Add Evolvable trait, modified to handle uninitialized properties.

This is a better central place for it than in Serde.