RFC: CI for all features

[sterraf]

The fix proposed by #633 addresses a problem not caught by CI: It seems that the current actions only cover the default features.

Is it reasonable to add --all-features to the current checks?

[sterraf]

After some experimentation in a fork, it seems that some features fail on Windows, specifically for crate gmp-mpfr-sys v1.6.5:

error: failed to run custom build command for `gmp-mpfr-sys v1.6.5`

Caused by:
  process didn't exit successfully: `D:\a\pallas\pallas\target\release\build\gmp-mpfr-sys-ccfb2c7f4dd9bfbf\build-script-build` (exit code: 101)
  --- stdout
  cargo:rerun-if-env-changed=GMP_MPFR_SYS_CACHE
  $ mkdir -p "D:\\a\\pallas\\pallas\\target\\release\\build\\gmp-mpfr-sys-9e2d6c486ca22066\\out\\lib"
  $ mkdir -p "D:\\a\\pallas\\pallas\\target\\release\\build\\gmp-mpfr-sys-9e2d6c486ca22066\\out\\include"

  --- stderr

  thread 'main' panicked at C:\Users\runneradmin/.cargo\registry\src\index.crates.io-1949cf8c6b5b557f\gmp-mpfr-sys-1.6.5\build.rs:1081:9:
  Windows MSVC target is not supported (linking would fail)

This could be sidestepped by separating in cases, but perhaps it might needlessly complicate the yaml file.