Calculate type signatures at compile time

[kennykerr]

WinRT use type signatures to generate GUIDs for generic interfaces. This update provides a const implementation for class and interface signatures, much like C++/WinRT, so that these signatures don't need to be redundantly included in the generated bindings as they can be derived from the other trait metadata included in the Rust bindings. This also helps to make the generated code more readable and concise.

For example, this generated trait:

impl ::windows_core::RuntimeType for TensorFloat {
    const SIGNATURE: ::windows_core::imp::ConstBuffer = ::windows_core::imp::ConstBuffer::from_slice(b"rc(Windows.AI.MachineLearning.TensorFloat;{f2282d82-aa02-42c8-a0c8-df1efc9676e1})");
}

Is simplified to this:

impl ::windows_core::RuntimeType for TensorFloat {
    const SIGNATURE: ::windows_core::imp::ConstBuffer = ::windows_core::imp::ConstBuffer::for_class::<Self>();
}

[kennykerr]

Trait bounds on const functions need Rust 1.62 or later: #2730