Rewrite to remove return statements where possible

[riftEmber]

Also, do that rewriting

[riftEmber]

I suppose that a single return statement should be allowed at the end of a function.

[riftEmber]

Work done for #37 disallows return statements that are inside any nesting, and disallows any statements to follow a return statement. This effectively implements the restrictions of this issue, except for the rewriting. Rewriting is a separate problem which this issue will now be about.