feat: lower aggregate return types

[mhasel]

Introduce a lowering stage for function and method calls

• Methods and functions returning strings will now be changed during a lowering phase (index and annotation) to have the an extra parameter to contain the return type.
  This used to be done on the codegen level but has now been moved to an earlier stage which would reduce the logic added to codegen to handle different types of returns.
• Introduce an "alloca" ast keyword that creates temporary variables on demand, this is needed to support re-writing the method and function calls with temporary variables.
  Note this currently causes a side effect in if/else blocks where an else condition will be evaluated even if the if condition is true, this will be fixed at a later point.
  For the same reason, WHILE and REPEAT..UNTIL loops are now lowered to WHILE TRUE loops, with a break-condition in the loop-body.
• Generics calls are now resolved during lowering/annotation and no longer in codegen.
• Methods are still defined as structs in the codegen, this will change to behave more like functions