Differentiate between statements and expressions

[niknetniko]

We now make a distinction between statements and expression.

We apply the following conventions:

• Statements are assumed to have no return value. This means that the "return" channel is always "ignored".
• Expressions have a default of "None". We do some stuff to ensure this also works in languages with "void" returns, such as Java, C# and C.

Fixes #372 and #400. Previous attempt at #374.

Most existing usages should be caught by the backwards compatibility (if you have an expected return value with a statement, the statement is treated as an expression instead). However, some uses might now change, but we'll fix those test suites as they occur.

[pdawyndt]

FYI: Concerning the use case where return values from expressions without a return attribute were previously "ignored" that would now fail if something other than "nothing" is returned. I consider this a good thing, as I think this was unwanted behavior in TESTed.