There should be an #opaque directive tag for structs

[Beefster09]

This is mainly for interfacing with foreign code.

An opaque struct has the following semantics:

• Has no size or alignment, so using size_of or align_of should be a compile-time error
• Cannot be given any fields (perhaps the brackets are optional?)
• Cannot be the element type of an array or slice
• Cannot be the key or value of a map
• Cannot be introspected with any of the struct_fields procs in core:reflect
• Passes intrinsics.type_is_struct checks and similar
• It is an error to use against intrinsics.type_has_fields and similar (not just a false result)
• Cannot be assigned or copied (as it has no size)
• Pointers to the type can be used just like a distinct rawptr