v0.20.0: Add midpoints, which was deprecated in Base. (#345)

* Add midpoints, which was deprecated in Base.

Currently not exported, to avoid symbol conflict.

* Add a methodless midpoints function for the definitions below.

* Changes suggested by @nalimilan.

Import midpoints for pre-v0.7, otherwise use unqualified symbol.

* Use more precise version

* Dividing by 2, to avoid accidental promotion.

Suggestion by @andreasnoack.