Prefer optional over convertible parameters

[kennykerr]

Thanks to #3359, it's now a lot easier to reason about some of these more nuanced type scenarios. The windows-bindgen crate has traditionally overused the Param trait to provide cover for various weird Windows API-isms but the Param trait tends to be quite confusing when you don't get the parameters just right and in particular for APIs that don't expect polymorphism. In some cases its necessary - specifically for COM or WinRT polymorphism - but in particular it was previously used for all Win32 handle types which was almost always unnecessary.

This update does two things:

• Avoid applying the Param trait for handle types
• Detect more optional parameters and use Option<T> for copyable parameter types

When combined this limits the fallout from the first and generally makes the API surface more coherent as optional parameters are actually optional and the vast majority of handle types, which aren't convertible, are simple type parameters.

In some cases where "handle polymorphism" is actually expected by some API you can simply use the Into or From traits to perform the conversion.

Fixes: #3320