Incomplete subtyping check on cells

[nathanielnrn]

#2085 introduces the possibility to pass in subtypes of components as part of invokes.

The well_formedness pass currently designates a component a as a subtype of b if a has at least the same ports (w.r.t width, name, direction, and attributes) as b. In practice this is implemented by comparison of two cells.

Unfortunately, this subtyping check is incomplete because it does not properly check for subtyping correctness of nested ref cells.

Meaning if component a expects a ref cell of a dyn_mem, and b has a ref cell of an axi_dyn_mem (a subtype of dyn_mem), b will still be considered a subtype of a even though it's not because the memory b expects is too specific.

Luckily, in practice this can't occur because nested ref cells are not allowed. When #2079 is implemented this can become a problem.

One thing to note on the implementation side is that a recursive check of subtyping is not immediately available from Cell comparison because only Components have information about cells. So one would need to access a Component from a Cell (this might already exist and be easy?), or change the information compile_invoke has access to so we can perform a comparison on Components directly.