[ArrayManager] Add SingleArrayManager to back a Series
xref https://github.com/pandas-dev/pandas/issues/39146#issuecomment-786741804

This implements a `SingleArrayManager` class for backing a Series, consistent with how we have a SingleBlockManager.

cc @jreback @jbrockmendel 
> is there a way to just have a SingleManager class that works for block / array. a SingleBlockManager after all is a single array.

I don't think so, AFAIK. A Block and an array have a quite different API (also, the SingleArrayManager is inheriting quite a bit from ArrayManager, so it's more different from SingleBlockManager than just the code added in the SingleArrayManager class here). 

There certainly might be some trivial properties that could be shared (like `.index`, and `.dtype` if if we add a `.array` to SingleBlockManager that is a shortcut to the Block's values) to reduce the size of the SingleArrayManager a bit. But note that right now it's not even 150 lines of code. 
 @jreback are you OK with this approach (see my response at https://github.com/pandas-dev/pandas/pull/40152#issuecomment-789240135) thanks @jorisvandenbossche 

ok generally ok with adding code, but more code == more complexity, so ultimately we want to simplify as much as possible. of course we are in the middle of new development on array manager so ok for now. but we want to consolidate.