The arena serves two purposes. It is a structure that is
the top-level state of the MPS, and as such contains a lot of fields
which are considered "global". And it provides raw memory to pools.

An arena belongs to a particular arena class. The class is selected
when the arena is created. Classes encapsulate both policy (such as
how pool placement preferences map into actual placement) and
mechanism (such as where the memory originates: operating system
virtual memory, client provided, or via malloc). Some behaviour
(mostly serving the "top-level datastructure" purpose) is implemented
by generic arena code, and some by arena class code.