[vernacexpr] Refactor fixpoint AST.

We turn the tuples used for (co)-fixpoints into records, cleaning up
their users.

More cleanup is be possible, in particular a few functions can now
shared among co and fixpoints, also `structured_fixpoint_expr` could
like be folded into the new record.

Feedback on the naming of the records fields is welcome.

This is a step towards cleaning up code in `funind`, as it is the main
consumer of this data structure, as it does quite a bit of fixpoint
manipulation.

cc: #6019

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