Typechecking - fast labels handling

[Mesabloo]

The easiest way of handling labels is to do it in two passes:

• a first pass where we just fetch all the labels along with their types
• a second pass where we perform typechecking on the whole program, statements each after the other

While this certainly would work, this is not very efficient (we need to traverse the whole AST twice). Maybe there exists a way to do both at the same time, while still keeping forward jumps.

A similar issue arises for the code generation, because on, for example, x86 instructions do not have a fixed width, impacting the labels addresses. But this will be discussed in time.