Introduce the mem_pool type which encapsulates all the information
necessary to manage a pool of memory. This change moves the existing
variables in fast-import used to support the global memory pool to use
this structure.
These changes allow for the multiple instances of a memory pool to
exist and be reused outside of fast-import. In a future commit the
mem_pool type will be moved to its own file.
Signed-off-by: Jameson Miller <jamill@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>