Implement Idx for u8, u16, u32

[samestep]

For my use case, I have a bunch of IndexVec<I, T> where I: Idx is a custom type, but I also need a few IndexVec<u32, T> as well. This PR implements Idx for the primitive types u8, u16, and u32.

The implementations for u8 and u16 should be uncontroversial. The one for u32 is a bit less clear since technically Rust does support 16-bit platforms, but it's the only one from this PR that I actually need 😅

[thomcc]

I'm not worried about 16 bit platforms, they can just not use that impl. It's been a while since I looked at this crate, but this seems fine, so long as you can make it compile and make tests pass.

[samestep]

Sounds good, thanks! I fixed the compilation error by just removing the impl for usize.

[thomcc]

Mind bumping the MSRV to whatever is needed that still works?

[samestep]

Hmm, I tried reproducing the CI failure locally via cargo +1.61.0 but was unable to do so. I suspect that the failure is unrelated to this PR, though; could you try allowing the CI run on #15 to see if that also fails?