RFC: Reintroduce nnz and nonzeros

Reintroduce nnz and nonzeros as discussed in #6769
Deprecate nfilled
Deprecate nonzeros for StridedArray and BitArray
find()/findnz() over sparse matrices no longer filters out stored zeros

Should `sparse()` allow stored zeros in its input, using an optional argument? Although this discussion started with ``nnz``, I feel it is leading towards better clarity on treatment of stored zeros, and overall I am feeling good about that.