Mass rename memory object variables.

Use shorthand:
AddressSpace -> vmas
MemoryObject -> vmmo

The VM prefix makes these a little more distinguishable in code.

sys/test.cpp

@@ -5,7 +5,7 @@
 int main() {
   dbgln("Testing");
   uint64_t vaddr;
-  check(ZAddressSpaceMap(Z_INIT_AS_SELF, 0, Z_INIT_BOOT_VMMO, &vaddr));
+  check(ZAddressSpaceMap(Z_INIT_VMAS_SELF, 0, Z_INIT_BOOT_VMMO, &vaddr));
   check(SpawnProcessFromElfRegion(vaddr));
   dbgln("Return");
   return 0;