Non-inline call to export function should be a simple warning

[vbgl]

Type-checking the following program produces a type error. This is twice wrong:

1. the program is well-typed
2. there is a canonical fix to make thy compiler happy (namely, tag the call to id as inline)

export fn id(reg u64 x) -> reg u64 { x = x; return x; }

inline
fn whynot(reg u64 a) -> reg u64 {
  a = id(a);
  return a;
}