Add assertions when implicitly returning from a function #2

Currently if a function implicitly returns undefined (by leaving the function without an explicit return statement) we won't generate a check for it, e.g:

function foo (): string {
  // no return
}

currently compiles to:

function foo () {
  const _returnType = t.return(t.string()); 
  // no return
}

We should insert a call to _returnType.assert(undefined) at these points.