Add CI script

[schteve]

Add basic CI script to check incoming pull requests and help maintain stability of master branch. Currently it only checks things that are working, but I left lines in place (commented out) for things that we plan to improve.

Currently checks:

• Build succeeds for the main library (warnings allowed)
  • freertos-rust
  • freertos-cargo-build
• Examples build (for the one that already works)
  • stm32-cortex-m4-blackpill

Not yet checked:

• fmt
• test
• clippy
• Build is clean (deny warnings)
• All examples build
• Examples build clean (deny warnings)

Some of the current issues under discussion should allow us to check builds for linux and win examples soon.

Toolchains are only set to nightly since the allocator_api still needs that.

Also related to recent discussions, it's not clear if running this on ubuntu-latest means that we might miss some build issues on windows (where build succeeds on linux but not on windows). For future investigation outside of this PR.

[schteve]

Also it looks like the CI script didn't run for this pull request - I'm not sure if this means something is wrong with the script (it worked on my fork), if it will work fine after being merged, or if there's some project level configuration needed to enable it. Any advice is welcome.

[schteve]

Merging this in the hope that it will run afterwards. Not clear to me what the rules are on running a CI script in a pull request but it seems to work fine on my fork at least.