kernel/process: Remove most allocation functions from Process' interface

In all cases that these functions are needed, the VMManager can just be
retrieved and used instead of providing the same functions in Process'
interface.

This also makes it a little nicer dependency-wise, since it gets rid of
cases where the VMManager interface was being used, and then switched
over to using the interface for a Process instance. Instead, it makes
all accesses uniform and uses the VMManager instance for all necessary
tasks.

All the basic memory mapping functions did was forward to the Process'
VMManager instance anyways.