Allow async property setter

[jaigak]

I have a property setter that needs to do some asynchronous operations. So I changed the return type from void to fire_and_forget however now it doesn't compile since the codegen in g.cpp attempts to return it which isn't possible in this case since fire_and_forget can't be returned in a void function.

[kennykerr]

Specifically, this is an edge case that only affects static setters when you are compiling with -opt. This already works in all other instances.

Here's a fix: #1063