Allow only `&'static str` in `Split`

[soenkehahn]

This narrows down the types allowed as arguments to Split (and therefore the types of values after % in macros). The reasoning is that

• this has hardly any downsides, since Split is meant as a convenience for working with string literals,
• it prevents surprising behavior in some corner-cases, for an example see https://github.com/matklad/cradle-of-tar.

For more discussion, see https://www.reddit.com/r/rust/comments/poqlo9/cradle_run_child_processes_with_ease/hd00yla/?utm_source=reddit&utm_medium=web2x&context=3.

Do we have any reasonable usecases where arguments to Split are not &'static str?

[casey]

I think this is reasonable. I can't think of any really important use cases for splitting non-static strings, and if someone has a use case, we can easily re-enable it.

[casey]

We should probably also only provide Input implementations for SplitWhitespace<'static>, Split<'static>, and SplitAsciiWhitespaceCopy<'static>.