BUG: SparseSeries concat results in dense
- [x] closes #10536
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

@jreback Added `core/concat.py` and moved all `concat` related utility funcs. Does this meet your intention?
 lgtm. any tests to move as well? (from core/common.py)?
 actually going to merge this. other things going to depend.
