Add TryFrom bounds for all integer primitive types to SimdIntElement

[danderson]

I skipped these in #302 because conditional conversion didn't seem great for typically performance sensitive simd code. However, it turns out that these conversions are very useful in tests especially, and even in regular code LLVM can often rule out failure and elide the branch.

Still no From impls other than bool, because that's the lowest common denominator across signed/unsigned and all the different sizes.

No change to SimdFloatElement: unfortunately they don't implement TryFrom for integers, so without num-traits's cast trait (which provides the equivalent of as conversion) we're stuck with the existing lowest common denominator.

[danderson]

Also added extra bounds so that TryFrom::Error must be Debug, so that unwrap and expect work without further bounds in the caller.

[danderson]

Hmm, the extra debug bounds cause failures that I don't get on my machine. Converting to draft while I figure it out.

[danderson]

Okay, fixed. The TryFrom impls all use either a core::num::TryFromIntError or Infallible as their error type, so I made TryFrom::Error's the useful traits both of those implement (Error implies Debug and Display).