Migrate golang.org/x/exp to the standard library

Migrate golang.org/x/exp/slices, golang.org/x/exp/maps, golang.org/x/exp/constraints and golang.org/x/exp/slog to the slices, maps, cmp and log/slog packages that absorbed them, and drop the golang.org/x/exp requirement once nothing needs it. Each rewrite is gated on the module's go directive, and a file using API the standard library never took — the numeric constraints, or the pre-2023 boolean comparators — is left for review. Run go mod tidy afterwards to sync go.sum.