lib: move cpuid::Set to cpuid_utils; prevent semantic subleaf conflicts

[gjcolombo]

Move the functionality in propolis::cpuid::Set into cpuid_utils, then tidy it up a bit by removing its for_regs and into_inner functions (the former was dead code and the latter is no longer needed).

Rework CpuidMap to prevent callers from using both CpuidIdent { leaf: x, subleaf: None } and CpuidIdent { leaf: x, subleaf: Some(y) } as keys in the same map (does CPUID with eax = x ignore the value passed in ecx or not?).¹ This requires a fair bit of new code to handle insert/remove and iteration. Add tests for these cases, including a property test to verify a larger number of iteration patterns than we could hope to write by hand.

Finally, add a theory statement and some additional function documentation to the cpuid_utils crate.

Footnotes

1. In practice the behavior depends on the order in which the conflicting entries appear in the vcpu_cpuid_entry array that Propolis passes to bhyve (the first one to match an incoming CPUID request wins); see bhyve's cpuid_find_entry. ↩

[hawkw]

Very nice! I had a very small suggestions and questions, but nothing big.

[iximeow]

i echo the proptest enthusiasm and appreciation for nice docs here 🙏

[gjcolombo]

Thanks as always for the reviews! Will merge once CI is green.