Add WinRT array support for windows-rdl

[kennykerr]

Building on #3861 and #3902, this update adds support for WinRT-style array parameters. WinRT defines three kinds of array parameters. There's the input parameter, which acts like a slice (or a view in C++). There's the output parameter, which is allocated by the callee - using CoTaskMemAlloc - and freed by the caller. And there's the return value, which is likewise allocated by the callee and freed by the caller in the same way. Here's an example:

#[winrt]
mod Test {
    interface ITest {
        fn Return(&self, input: [i32]) -> [i32];
        fn Out(&self, input: [i32], output: &mut [i32]);
    }
}

And here's the ECMA-335 representation:

.class interface public auto ansi abstract windowsruntime Test.ITest
{
	.method public hidebysig newslot abstract virtual 
		instance int32[] Return (
			[in] int32[] input
		) cil managed 
	{
	}
	.method public hidebysig newslot abstract virtual 
		instance void Out (
			[in] int32[] input,
			[out] int32[]& output
		) cil managed 
	{
	}
}

Win32 offers a different set of array primitives. I'll deal with those separately.