Metadata in decorators

[alcides]

The idea of this feature is two-fold:

Firstly, we want to support multiple decorators (e.g., multiple minimize targets that just append to a list of objectives).

@dec1(...)
@dec2(...)
def f : Int := 3;

Secondly, we want to save information together with the AST ("eg: what is the list of fitness functions, what functions to ignore, etc).

How to implement:

• Decorators need to accept a metadata dictionary, and return the new metadata dictionary.
• When decorators are being applied for each function, the new metadata is passed to the next decorator.
• Between function definitions, the metadata dictionary is also passed on to the next one.
• Metadata needs to be passed on to the synthesis procedure.