This commit moves the KResult and KResultOr objects to Kernel/API to
signify that they may now be freely used by userspace code at points
where a syscall-related error result is to be expected. It also exposes
KResult and KResultOr to the global namespace to make it nicer to use
for userspace code.
...and also RangeAllocator => VirtualRangeAllocator.
This clarifies that the ranges we're dealing with are *virtual* memory
ranges and not anything else.