Rolling up PRs in the queue

Closes #14590 (librustc: Implement overloading for the call operator behind a feature)
Closes #14611 (std: Remove the as_utf16_p functions)
Closes #14694 (Num cleanup)
Closes #14760 (Add --color to test binary options)
Closes #14763 (std: Move dynamic_lib from std::unstable to std)
Closes #14766 (Add test for issue #13446)
Closes #14769 (collections: Add missing Default impls)
Closes #14773 (General nits)
Closes #14776 (rustdoc: Correctly classify enums/typedefs)