Add a @defer function

[dullbananas]

With this built-in function, it would be possible for functions to accept @defer or null as an argument to prevent forgetting to defer something.

For example, ArenaAllocator could be used like this:

let foo = ArenaAllocator.init(@defer, alloc);

Manually calling deinit:

let foo = ArenaAllocator.init(null, alloc);
foo.deinit()

The init function calls the provided defer function, defer_arg, like this (requires closure stntax):

if (defer_arg) |defer| {
    defer(fn() void {
        self.deinit()
    });
}

[InKryption]

Duplicate of #782.