Extend the wrong-kind mask guard to dup and whole-object update - #614

A.dup(mask=v.S) and C(mask=v.S) << A assign with C(mask)[...] = A,
which builds a __setitem__ expression, so the strict mask-kind check
in BaseType._update did not apply and a Vector mask on a Matrix
output reached GrB_Matrix_assign, leaking a raw cffi error:

    TypeError: Error calling GrB_Matrix_assign: ... initializer for
    ctype 'struct GB_Matrix_opaque *' must be a pointer to same type,
    not cdata 'struct GB_Vector_opaque *'

Validate the mask kind in the Matrix-valued branch of
Matrix._prep_for_assign, mirroring the guard the Vector-valued
branch already has. This also covers C(v.S)[:, :] << A and the
submask form C[:, :](v.S) << A. Row and column assignment with a
Vector mask (C(v.S)[0, :] << w and friends) is handled in earlier
branches and remains valid.

The guard keys on mask.parent.ndim rather than exact type, the same
idiom _check_mask uses, so a Vector subclass's mask is caught too. Two
pre-existing guards in the Vector-valued branch and one more in the
scalar path still use exact-type checks and carry the same subclass
hole; they predate this commit and are left for their own change.