Opening a VFS handle previously involved allocating from the global heap
at each VFS file system. By amending open with an allocator argument,
dynamic allocation can be partitioned.
A new close method is used to deallocate open handles.
Issue #1751
Issue #1891
This patch changes the top-level directory layout as a preparatory
step for improving the tools for managing 3rd-party source codes.
The rationale is described in the issue referenced below.
Issue #1082