Add necessary functionality for migrating kernel paging code to RMM

See merge request !6

Currently, this uses a relatively naive method of simply scanning the
512 entries for the PRESENT flag. But, unless the optimizer cannot, it
can be reduced to calculating the bitwise OR of every entry and then
checking that.

If this turns out to be too slow, which it might be when unmapping lots
of pages, then we can (1) either fall back to using a counter like the
old paging code did, or even better (2) use the now-1:1 grants tree to
check if it became empty. Putting the grants code in RMM might be
suboptimal, so instead we can add "unmap_range" and have the kernel
paging code take the offset of the next grant, if any, and then possibly
unmap entire P1s/P2s/P3s -- whatever is in the page tables within that
range.

Note that I am fairly certain that method (1) was the cause of the
visually notorious orbital memory corruption bug.

Update to latest toolchain.

See merge request !5

Add Arch::virt_is_valid

See merge request !4

All there is left now is AArch64.

Fix possible UB by implementing Copy and Clone manually for BuddyEntry.

See merge request !1

This fixes a warning that may in the future become an error, about the
possibility for unaligned references, since the derive macros apparently
rely on creating references to fields. Unaligned references are direct
UB.