Merge `Type` and `Term` in `hugr-core`.

[zrho]

#2309 merges TypeArg and TypeParam into one data structure in hugr-core. We should further merge Type into Term.

Related Issues

• The code currently special cases lists of Types to deal with "row variables". To enable a clean merger, we'd need to get rid of row variables first (see No more row variables #2310).
• The capability to define custom Types would be extended to custom Terms that aren't runtime types (which would solve Allow custom TypeArgs and TypeParams. #2296).
• Merging Types and Terms would allow runtime types to profit from the sharing enabled by reference counted terms (see feat!: Reference counted terms #2338). Runtime types are the primary source of heavily duplicated terms, so this could have a large performance benefit (especially on model deserialisation).