Iterator::collect should ideally support conversions

[danakj]

An Iterator<const std::string&> can not be collected into a Vecstd::string, because it requires FromIterator<std::string> to be true for Iterator<const std::string&> which it's not. The from_iter() impl needs to accept an IntoIterator<std::string> which is not matching Iterator<const std::string&>'s implementation which returns itself.

An example in subdoc:

  run_options.macro_prefixes =
      sus::iter::from_range(option_include_macro_prefixes)
          .cloned()
          .collect<sus::Vec<std::string>>();

This should really work without cloned() ideally, since it's a copy constructible type.

[danakj]

Some thoughts from the PR #425:

The issue is that IntoIterator (and thus collect) requires the into_iter() function's returned Iterator::Item type to be exactly the same as the type specified in IntoIterator. But I believe it would be better here to match an Iterator::Item type if its convertible-to the type in IntoIterator.

Some options:

• Maybe collect needs to use a more lenient version of IntoIterator, and perform a map itself when the types don't match.
• Maybe Iterator can match a concrete type if its Item is convertible-to T since next() returning a T will be valid in code that wants something T converts to as well.

Would the latter then automatically make IntoIterator more lenient? I think so, but didn't look carefully yet.