Simplified parameter bindings and type projection

[kennykerr]

This update simplifies a few notable areas of code generation and type projection:

• The distinction between compile- and run-time convertible parameters has been removed from the generated parameter bindings and pushed down into the type traits themselves. Practically that means that there is no longer the confusing difference between the IntoParam and TryIntoParam constraints.

• The ComInterface and Interface traits have been combined. The distinction only matters for a small number of non-IUnknown interfaces, but the divide caused a great deal of complexity. The single Interface trait now handles both. The cast method will however panic if called from a non-IUnknown interface for obvious reasons. The branching is a compile-time decision so the unused branch should be compiled away.

• A lot more of the boilerplate code around COM interface declarations is now hidden behind simplified macros which makes the code a lot easier to read and substantially more concise.

This also fixes #2759