The test harness code is useful across architectures. Therefore, move it
out of the arm64 directory so it can be used by other architectures
implementing paging unit-tests.
We currently define the MAP_SHIFT for a page-table level in two places,
`TableMapper` and `PgTable`. Store only one shift trait constant value
in the `PageTableEntry` trait.
Any traits which are architecture agnostic should be available to be
implemented by other architectures. Move them out from the arm64 module
into a non-architecure specific location.
Move PtePermissions to paging module which is feature-gated behind
`paging`.
Also move all AddressSpace related functionality behind the `proc_vm`
feature gate.
