Editable typedefs: type parameters

[georgefst]

From "not in spec" section of #267:

Another case to handle is a student adding a parameter to a type: data T x = A -> data T x y = A.

Note that any fresh parameter is, of course, initially unused.

We may want an easy way to allow making a type more or less polymorphic. e.g. converting between data IntList = Empty | Cons Int IntList and data List a = Empty | Cons a (List a).

Note also that if we add or remove parameters, we change a type's kind, and will need to update other typedefs which depend on it.

With #401 being closed, I will use this issue to also track deletion of parameters.

[brprice]

@georgefst Could you update us on the current status of this?

IIUC, we support adding (and deleting) a parameter but only when the type is unused?

[georgefst]

Like #401, I forgot to come back to this issue when this was partially implemented in (the somewhat mis-titled) #1063, adding support for these edits in unused types.

For supporting used types, we need to consider how to fix up type-level expressions where the kind of a type has changed. It might be that this kind/type case is not significantly harder than the already-implemented type/term case. We just have not yet attempted to enumerate all the possible interactions in the manner of #267. Note that when we do so, we should consider deletion of parameters (which overlaps with #401).

Also, note that while the opening comment talks only about adding and removing params, we also support editing the kinds of parameters (#1095), and need to consider how to handle these changes for in-use types.

We also need some unit tests. AddTypeParam and DeleteTypeParam do not appear anywhere in the test suite.