Improve CI coverage

[IsaacWoods]

We've had quite a few issues recently due to failures of CI to do a good job for us.

We should improve coverage for some areas:

• Do we successfully compile on the most recently nightly?
• Test full feature matrix
• Do we successfully compile on a MSRV?
• Do we successfully compile for 32-bit
• (maybe) do we pass tests on a little x86_64 kernel (both BIOS and UEFI ideally) (can also serve as an example kernel cc Kernel Example with ACPI #125)

cc: #146 #147 #133 #160