Semantic analysis should reject ret, next, and break expressions in defer statements

[alexrp]

Consider:

{
    defer ret :ok;

    raise err MyError {};
}

We clearly shouldn't allow this as the defer statement effectively becomes a weird kind of try/catch-like mechanism.

[alexrp]

Also:

• ret expressions should be rejected if there's no parent function to bind to.
• raise expressions and error-propagating calls, notably, should be allowed in defer statements.
• We should attach a list of ret expressions to FunctionDeclarationSemantics/LambdaExpressionSemantics.