Update spec to include missing cases for bounds-safe interface assignment compatibility

[kkjeer]

The current rules for assignment compatibility of expressions involving bounds-safe interfaces are missing two cases.

The first case is related to assignments involving nested pointers. An assignment *p = e can pass type checking if:

• p has an unchecked pointer type and a bounds-safe interface, and
• e has a checked pointer type, and
• the referent type of e is assignment compatible with the referent type of the referent type of p

The second case is related to assignments of an unchecked pointer with an _Nt_array_ptr bounds-safe interface to an _Nt_array_ptr variable. An assignment v = e can pass type checking if:

• v has type _Nt_array_ptr<T>, and
• e has unchecked pointer type and a bounds-safe interface of _Nt_array_ptr<U>, and
• the referent types T and U are assignment compatible.