Limit mutual recursion in core Fortran AST types

[raehik]

The Fortran syntax representation we use is highly mutually recursive. To summarise the core types:

• a ProgramFile contains many
• ProgramUnits, which contain
• Blocks, which contain
• Statements, which contain
• Expressions.

However, as of 2022-05-11, Expressions are mutually recursive with ProgramUnits. This can introduce surprises, and makes instance derivation awkward. In particular, generic derivation likely won't work unless every type up to ProgramUnit also has an instance derived.

It would be more sensible to limit the two-way recursion where possible. Note there might be some unexpected behaviour by doing so. For example, Expressions are only connected to Statements through one constructor -- altering a data type stored in there removes the mutual recursion, but caused issues with block/flow analysis #221 .