Use `check_is_assignable_to_using_subtype` in
`is_polymorphic_type_assignable`.
The polymorphic procedure in #486 can now also be written without the
cast:
```odin
print_entity :: proc(e : ^$E, p : proc(^$T) = print_base) { p(e); }
```
Modify `check_type_specialization_to` to require exact values
to be equal when called with constant basic types. This also
now allows procedure group members to differ only by constant
value specializations. See the further example in the issue.
Joseph Battelle