Support Table mixin columns

This is essentially the same as #1833, #1835, #1839 but will be done in the framework of #2790 as a follow-on once that is merged.

The starting point here will be fed00dc5f6d6d9529ed8f073c95346eb176fdfa6, which *removed* the proof-of-concept changes showing that mixin columns would be straightforward in the new column-based Table structure.   In this issue (and eventual PR) we can define the right way to support mixin columns like Quantity, Time, and SkyCoord.  The original proof-of-concept used a some simple class / instance variables, but @mhvk has proposed using an abstract base class.

Closes #1833
Closes #1835
Closes #1839
They get used in `io.ascii`.  I was using the rule that functions which are used in other astropy sub-packages should be "public" (not starting with `_`), although not necessarily exposed in the official public API as part of table.  The problem here is in handling an experimental interface that is quite likely to change, but yet requires use of these methods to fully function.  Any user code would need to use them by importing from `table.column`.  I guess at the least I can put in proper docstrings.