Allow the name and value in set_string to be 2 different types

[kerosina]

Currently, windows_registry::key::Key::set_string takes one generic type T which implements AsRef<str>, which is the type of the arguments name and value. This signature is too restrictive, since both arguments must be of the same concrete type, while it should accept, for example, key.set_string(String::from("Name"),"Value").

This PR adds a new generic type U, with the same trait bound as T and which is the type of value, thereby allowing the aforementioned situation and similar cases where the 2 arguments have different concrete types that fulfill the same trait bound.