Merge pull request #2021 from nordlow/warn-unused-returns

Issue 3882: Use cast(void) instead of value capture
Make std.utf.toUTFz safe and pure
Merge pull request #2023 from tom-tan/safe-pure-std-utf-toUTFz

Make std.utf.toUTFz safe and pure
Issue 3882: Use cast(void) instead of value capture on calls to assumeSafeAppend