Kernel: Factor address space management out of the Process class

This patch adds Space, a class representing a process's address space.

- Each Process has a Space.
- The Space owns the PageDirectory and all Regions in the Process.

This allows us to reorganize sys$execve() so that it constructs and
populates a new Space fully before committing to it.

Previously, we would construct the new address space while still
running in the old one, and encountering an error meant we had to do
tedious and error-prone rollback.

Those problems are now gone, replaced by what's hopefully a set of much
smaller problems and missing cleanups. :^)

Kernel/FileSystem/ProcFS.cpp

@@ -317,8 +317,8 @@ static bool procfs$pid_vm(InodeIdentifier identifier, KBufferBuilder& builder)
         return false;
     JsonArraySerializer array { builder };
     {
-        ScopedSpinLock lock(process->get_lock());
-        for (auto& region : process->regions()) {
+        ScopedSpinLock lock(process->space().get_lock());
+        for (auto& region : process->space().regions()) {
             if (!region.is_user_accessible() && !Process::current()->is_superuser())
                 continue;
             auto region_object = array.add_object();