ci: add windows/macos to clippy to reproduce compilation error on windows

[delta1]

this PR adds windows and macos to the clippy job to reproduce the compilation failure in #186

also fixes some clippy warns/errs

[delta1]

I will remove the clippy commit

[delta1]

closing for https://github.com/BlockstreamResearch/simplicity/pull/211

[delta1]

Is it worth having this change in CI to check macos/windows compilation?

[delta1]

rebased

[uncomputable]

Depends on #191. Let's merge this PR once CI runs.

[uncomputable]

@delta1 Feel free to rebase

[delta1]

@uncomputable rebased. windows ci failure is fixed upstream and in #192

[uncomputable]

Let's rebase on top of the merged #192 to fix CI.

[delta1]

@uncomputable done. macos failure looks spurious

[apoelstra]

@uncomputable this crate currently doesn't build with the elements feature and apparently our CI doesn't check this. It was broken in #189.

I can't run my local CI, or even cargo test, until this is fixed.

[uncomputable]

@apoelstra I thought our CI would catch this, but apparently it doesn't. master doesn't build on my machine :/

Let's fix master in a separate PR.