Warn on `build_assert` calls without `#[inline(always)]`

[nbdd0121]

It could ended up being not inlined when clippy is in use.

[ShigrafS]

@nbdd0121 I would like to work on this.

[nbdd0121]

This feature is more complicated than the name might suggest. It is a non-local analysis because the requirement propagates to callers, too. I would not recommend to work on this as "good first issue".

[ShigrafS]

In that case, do you think I can start with #6 ?

[nbdd0121]

Sure, feel free to give it a try. If you want something simpler, you can also try #9.