Move time conversion funcs to tslibs.conversion
Eventually a bunch of `_TSObject` logic belongs in `tslibs.conversion`.  To start off small, this just moves a few out-of-place functions from `_libs.lib` and `_libs.index`.

The changes that are not cut/paste:

In `_to_i8`: `tslib._delta_to_nanoseconds(offset)` became `int(offset.total_seconds() * 1e9)`
In `array_to_timestamp` and `time64_to_datetime`: `for i from 0 <= i < n:` became `for i in range(n):`

Right now the functions moved from `lib` are imported with a `# noqa`. If OK, I'll follow-up by updating the imports elsewhere.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
