Merge #271
271: Allow warnings during local development r=alistair23 a=jrvanwhy
This keep's CI's behavior where it errors if libtock-rs has warnings.
This is a second attempt at PR #270. We can't specify `RUSTFLAGS` directly because doing so overrides all `rustflags` configurations in `.cargo/config`. Those `rustflags` configs are important because they configure relocation and set the linker script. Instead, I append the `"deny warnings"` config to `.cargo/config` in CI itself.
I tested this by opening two PRs in my fork: [one to test that this PR still builds](https://github.com/jrvanwhy/libtock-rs/pull/15), and [one to test it fails if a warning is present](https://github.com/jrvanwhy/libtock-rs/pull/13).
I am not terribly happy with this solution (feels really hacky rather than kinda hacky), so any alternative implementation ideas would be appreciated!
Co-authored-by: Johnathan Van Why <jrvanwhy@google.com>
