This commit eliminates the global new/delete overrides that were causing
compatibility problems (the Allocators.(h/cpp/inl) files have been
completely removed). The GPOS `New()` macro is retained and works the
same way, but has been renamed `GPOS_NEW()` to avoid confusion and
possible name collisions. `GPOS_NEW()` works only for allocating
singleton objects. For allocating arrays, `GPOS_NEW_ARRAY()` is
provided. Because we no longer override the global delete,
objects/arrays allocated by `GPOS_NEW()` and `GPOS_NEW_ARRAY()` must now
be deleted by the new functions `GPOS_DELETE()` and
`GPOS_DELETE_ARRAY()` respectively. All code in GPOS has been
retrofitted for these changes, but Orca and other code that depends on
GPOS should also be changed.

Note that `GPOS_NEW()` and `GPOS_NEW_ARRAY()` should both be
exception-safe and not leak memory when a constructor throws.

Closes #166