implement sizeof for SubArray

[JeffBezanson]

This surprised me. Is there any reason not to have this method?
Found while working on #25241.

[andreasnoack]

I guess there has generally been some abstraction confusion about when something is an outer structure or the actual thing. E.g. when discussing copy for sparse arrays. When discussing copy, you mentioned that these kinds of arrays shouldn't be considered an outer structure (which is the interpretation I also prefer) and it seems to be the same question now for sizeof. Hence, we should probably also have sizeof(A::SparseMatricCSC) = sizeof(A.colptr) + sizeof(A.rowval) + sizeof(A.nzval). In any case, it would be great with a short discussion of this in the manual to use as reference for future discussions.

[JeffBezanson]

One thing we can add to the doc string is that if something can be written out with write, then sizeof should give the number of bytes that would be written. Under that idea, SubArray definitely qualifies but SparseMatrixCSC could still go either way.