Support aggregate constant initialization

[kennykerr]

The Windows API has a few constants that are more than just plain primitive literals. This update picks up support for the new Constant attribute that provides an initializer syntax for such structs. You can see this in action in the SECURITY_NT_AUTHORITY constant that is now present in both the windows and windows-sys crates. It turns the following metadata:

[Constant("{0, 0, 0, 0, 0, 5}")]
public static SID_IDENTIFIER_AUTHORITY SECURITY_NT_AUTHORITY;

...into this Rust constant:

pub const SECURITY_NT_AUTHORITY: SID_IDENTIFIER_AUTHORITY = SID_IDENTIFIER_AUTHORITY { Value: [0, 0, 0, 0, 0, 5] };

The next update to the Win32 metadata will introduce a bunch of additional constants that were previously missing. The constant parser is a bit simplistic but will do for now until I have better support for metadata generation and can write isolated tests for it.