Inline struct/tuple returns

[N00byEdge]

Currently, you can only make normal structs that can be returned through RLS and not normal returns.
We should add declarations for inline structs which can be returned through (multiple) registers but not RLS:

const T = struct inline {
  a: u64,
  b: u64,
};

fn f() T {
  return .{
    .a = 5,
    .b = 3,
  };
}

->

f:
  mov rax, 5
  mov rbx, 3
  ret