address obtained from system malloc/realloc without holding the GIL.

When the vector of arena base addresses has to grow, the old vector is
deliberately leaked.  This makes "stale" x-thread references safe.
arenas and narenas are also declared volatile, and changed in an order
that prevents a thread from picking up a value of narenas too large
for the value of arenas it sees.

Added more asserts.

Fixed an old inaccurate comment.

Added a comment explaining why it's safe to call pymalloc free/realloc
with an address obtained from system malloc/realloc even when arenas is
still NULL (this is obscure, since the ADDRESS_IN_RANGE macro
appears <wink> to index into arenas).