Implement Clone for iterator adaptors using functions

[bluss]

impl Clone wherever the underlying iterator and the predicate / mapping
function implement Clone. For example bare functions impl Clone + FnMut,
so such functions used with .map() will produce a clonable iterator.

With this change, str's .bytes() iterator becomes clonable.

Add a single test that execises .clone() on a Map.

[rust-highfive]

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[Gankra]

I believe this is already accomplished by #19827

[bluss]

Thank you for the pointer, I think this PR accomplishes it better.

[bluss]

Superseded by the other PR.