- In general, a generic on the RHS can accept anything on the LHS...
- ...but a non-generic type on the RHS cannot accept a generic on the
  LHS...
- ...buf if either carry a type signature, we should take advantage of it...
- ...mapping type capture to type capture until we find any non-generic
  type (or Mu).
- A return type must not exist on the LHS if one does not exist on the
  RHS, otherwise it should defer to ACCEPTS as with parameters.

Unlike before:

- Spare any unnecessary &EVAL nonsense.
- Ensure that generic doees, parents, and a complicated blend of the two
  can typecheck.
- Ensure we can access the doees list before composition (as
  typechecks may depend on it).

Because the cons of picking Array for type objects (need to decont for
`=:=`. not a normal array otherwise) outweigh the pros of mutability
here.