Functions with return type should return last expression

[JSAbrahams]

Current Issue

Unless we explicitly end a function with return, it does not return anything.

High-level description of the feature

In Mamba, a function that ends with an expression returns that type as long if the signature states as such.

Description of potential implementation

See #165

[JSAbrahams]

Catagorized this as bug since a few tests already assume this behaviour partially (in the type checker). The generate code stage however does not yet deal with this, so the reference (*_check.py) files always assume no return if not explicit in Mamba.

[JSAbrahams]

Forgot about match arms

[JSAbrahams]

Hm, and ternary expressions?

[JSAbrahams]

Well, it is actually not used in the generate stage anymore. Might be useful though if we want to use an if as an expression in other places (not perse in a function). I'll leave this issue open so I don't forget to make a separate issue for the above.