Duncan Coutts authored Jan 08, 2023 and Marge Bot committed Apr 03, 2024

The implementation is eventually going to need to use more private
things, which will drag in unwanted includes into IOManager.h, so it's
better to move the impl out of the header file and into the .c file, at
the slight cost of it no longer being inline.

At the same time, change to the "switch (iomgr_type)" style.