remove force_eval! macro, prepare more const functions, fix clippy, and add clippy to CI

[phip1611]

This MR is intended to be a preparation for #262.

• fix all clippy warnings (or properly ignore, where sensible)
• add clippy to CI
• remove the force_eval! macro with something more const friendly

Regarding the removal of the force_eval!-macro:
Fomment from @Amanieu from the other PR:

I am generally in favor of the PR!

The volatile reads are a bit of a hack. I believe they were used for 2 purposes:

• To set the appropriate FP status flags (overflow, underflow, etc) in the FP status register. We really shouldn't bother with this since we don't guarantee any particular value for flags.
• To work around FP precision issues on 32-bit x86 which uses 80-bit float precision internally. This is unfortunately an unavoidable issue on x86 since it has pretty broken FP support.

[phip1611]

ping @Amanieu - I'm not sure whether the CI error might be unrelated.

[Amanieu]

The test failures don't happen on master, this is definitely related to your changes.

[Amanieu]

You can reproduce the test failures locally with cargo test --release.