Optimise Msrv for common one item case

[GnomedDev]

Currently, Msrv is cloned around a lot in order to handle the #[clippy::msrv] attribute. This attribute, however, means RustcVersion will be heap allocated if there is only one source of an msrv (eg: rust-version in Cargo.toml).

This PR optimizes for said case, while keeping the external interface the same by:

• Storing RustVersion inline for the common one element case.
• Swapping to ThinVec to shrink Msrv to 16 bytes, from 24, to optimize for the zero and one element cases.

changelog: none

[rustbot]

r? @Centri3

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[flip1995]

r? @Jarcho as they looked into this most recently.

[Jarcho]

I don't see any reason to use ThinVec here over SmallVec which would be the same size and be simpler.

[GnomedDev]

SmallVec would be 24 bytes inline, as it stores the capacity and length inline in the heap allocated variant. I also didn't think it was already in the dependency tree, which this solution keeps the same. If you want to undo the ThinVec win, I can swap to SmallVec, but there is a benefit to this method.