CLN: Move boxing logic to BlockManager
- [x] closes #12741 closes #12772 closes #12469
- [x] tests added / passed
  - [x] `map` related boxing
  - [x] `quantile`
- [x] passes `git diff upstream/master | flake8 --diff`
- added `Series.asobject` to return boxed `object` values
- remove `common/i8_boxer()`, `Series._maybe_box()` (only used by `.quantile()`)

If base direction looks OK, going to add some tests based on the spec.
