Start to centralise the I/O manager hooks from other bits of the RTS

It is currently rather difficult to understand or work with the various
I/O manager implementations. This is for a few reasons:

1. They do not have a clear or common API. There are some common
   function names, but a lot of things just get called directly.
2. They have hooks into many other parts of the RTS where they get
   called from.
3. There is a _lot_ of CPP involved, both THREADED_RTS vs !THREADED_RTS
   and also mingw32_HOST_OS vs !mingw32_HOST_OS. This doesn't really
   identify the I/O manager implementation.
4. They have data structures with unclear ownership, or that are
   co-owned with other components like the scheduler. Some data
   structures are used by multiple I/O managers.

One thing that would help is if the interface between the I/O managers
and the rest of the RTS was clearer, even if it was not completely
uniform. Centralising it would make it easier to see how to reduce any
unnecessary diversity in the interfaces.

This patch makes a start by creating a new IOManager.{h,c} module. It is
initially empty, but we will move things into it in subsequent patches.

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