Added std.algorithm.safeLess for safe signed/unsigned comparison; rewrote min/max
Add `@nogc` attributes to `std.utf`.

Also remove GC allocation from `byDchar.front` as it caused all `by*` ranges be not-`@nogc`.
Merge pull request #2198 from lionello/safeLess

Added std.algorithm.safeLess for safe signed/unsigned comparison
Merge pull request #2331 from denis-sh/add-`@nogc`-to-`std.utf`

Add `@nogc` attributes to `std.utf`.