(Closes #2642, initial step towards #2643) Implement extends(type), procedure in derived type, class in declarations.

[JulienRemy]

This adds the following object- and inheritance-oriented features:

• an extends attribute in StructureType for inheritance of derived types, which avoids relying on UnsupportedFortranType and its declaration: str attribute,
• a procedure_components attribute in StructureType for procedures in a contains block of a derived type, including visibility and initial value,
• support for class (versus type) in routine declarations, by introducing a boolean attribute in DataTypeSymbol.

Note that as this makes some derived types supported there is for now a workaround using FortranWriter in both GOceanKernelMetadata and LFRicKernelMetadata, which rely on UnsupportedFortranType().declaration and fparser, see #2643.

[JulienRemy]

This will require me to edit the documentation, including (but maybe not limited to) https://psyclone-dev.readthedocs.io/en/latest/psyir_symbols.html#datatypes

[JulienRemy]

@arporter @sergisiso This works fine in practice on our project.
I’ll need to add some more specific tests, see what’s up with the master branch conflicts (Aidan’s routines things?) and refactor this thing for sure so it’s going to stay a draft until next week at least.
But if you have time for a broad overview at some point I’m definitely interested in it :) Don’t bother with the details, but if something doesn’t agree with what you’d want in there, feel very free to point it out and I’ll rework it. Most especially: I’m not so sure the way I’m getting the fparser attributes in the frontend is very clean at times.