CI: Lint the crate and examples

[tcharding]

We are not currently running the linter in CI, do so.

[apoelstra]

Will $clippy --all-targets get the same effect?

I think we should try running

clippy --all-features --all-targets
clippy --all-targets
clippy --no-default-features --all-targets # maybe with --features=no-std if we require that here

Which I think will cover the examples (though let me check..) and then gets us a bit of feature-matrix coverage as well.

[apoelstra]

Yeah, confirmed that --all-targets will flag let x = u8::from(10u8) in an example program.

[tcharding]

Interesting, then we (I) got it wrong in rust-bitcoin - I thought I had checked this before.

IIUC running the linter with different feature flags only adds checking unused imports when features are turned off, and while its nice to have these correct I'm not sure its worth bothering contributors with failing CI runs for it?

EDIT: In hindsight this comment is dopey as hell :)

[tcharding]

I was wrong we get real errors with cargo +nightly clippy --no-default-features --features=no-std --all-targets -- -D warnings that we don't get with --all-features - WIN.

Here is an example

error: this expression creates a reference which is immediately dereferenced by the compiler
   --> src/lib.rs:757:37
    |
757 |             fn deref(&self) -> &T { &self.lock.deref() }
    |                                     ^^^^^^^^^^^^^^^^^^ help: change this to: `self.lock.deref()`

[tcharding]

Now includes two clippy fixes up front and uses the three feature gate combinations suggested.

[apoelstra]

ACK a410c06