API: pandas.api
In xref #13147

established a bit of a public api in `pandas.api`; ATM this only contains the type introspection routines.
## 1st question

Some disagreement on whether we should not do this, and rather just expose `pandas.types` directly.

I think `pandas.api` is actually very important because:

1) limits scope of what we choose to expose in the future; its not simply 'everything' that isnt nailed down (the case now). This does change pandas to make everything by default private, EXCEPT what is explicitly public.

2) a single entry point for other packages to use the pandas public API that is known / stable / maintained (will with c-API as well)

3) provide consistent naming externally (and allow us to fix / hide internally as needed)

4) namespaced. I only import _what_ I need as a user / other package, rather than _everything_
## ~~2nd question~~

 as discussed [here](https://github.com/pydata/pandas/pull/13147#discussion_r70594911), should these deprecated API functions should be `DeprecationWrarning` rather than `FutureWarning`?
-> done in https://github.com/pydata/pandas/pull/13990

Ideally we should resolve this before the end of 0.19.0 (or remove it).
