Add Win32/WinRT type system distinction for windows-rdl

[kennykerr]

Although RDL and windows-rs in general supports both Win32 and WinRT types and they share the same ECMA-335 format for metadata definitions, WinRT types are distinct and must be identifiable as they conform to the stricter type system understood by WinRT language projections. In ECMA-335 this is represented by the WindowsRuntime type attribute in the TypeDef table.

This update adds the ability (and requirement) that types are unambiguously defined as either WinRT or Win32. Eventually, I'll add validation to ensure that WinRT types only refer to other WinRT types, while Win32 types can refer to both WinRT and Win32 types as needed. For now, this is just a big upgrade to the RDL reader and writer to accommodate this distinction.

Each type must have either a winrt or win32 attribute as follows:

mod Test {
    #[winrt]
    struct A {}

    #[win32]
    #[repr(i32)]
    enum B {}
}

This will produce the following metadata:

.namespace Test
{
	.class public sequential ansi sealed windowsruntime Test.A
		extends [mscorlib]System.ValueType
	{
	}

	.class public auto ansi sealed Test.B
		extends [mscorlib]System.Enum
	{
		.field private specialname rtspecialname int32 value__
	}
}

Alternatively, types can inherit this attribute from the surrounding module for convenience:

#[win32]
mod Test {
    struct A {}
    
    #[repr(i32)]
    enum B {}
}

In this case, both types will be defined as Win32 types. This default can also be overriden:

#[win32]
mod Test {
    struct A {}

    #[winrt]
    #[repr(i32)]
    enum B {}
}

In this, A inherits the win32 attribute while B overrides this default.

This work builds on #3861.