549be547ac primitives: Add must_use (Tobin C. Harding)
Pull request description:
Enable lint `clippy::return_self_not_must_use` and add attribute `must_use` as required.
Also run the linter with `clippy::must_use_candidate` enabled and manually check every warning site.
Done as part of #3185
ACKs for top commit:
apoelstra:
ACK 549be547accdb13037bf3ef60310ca30d045dce0; successfully ran local tests; nice! will one-ACK merge
sanket1729:
ACK 549be547ac
Tree-SHA512: b22a6849fd0f4a7b65e1a9816efd47d411dcf2a5d5d46ae75b2b4d2389d3c9f46ab271314b112de9cd6fdc52cac7b53a632642e9bd90092d7065a8646e1362ec