* spot/priv/allocator.hh, spot/priv/Makefile.am: add a STL-compliant
  allocator based on spot::fixed_size_pool
* spot/misc/fixpool.hh, spot/misc/fixpool.cc, spot/misc/Makefile.am:
  refactor the existing spot::fixed_size_pool
* spot/ltsmin/ltsmin.cc, spot/twa/twaproduct.cc: reflect changes in the
  interface of spot::fixed_size_pool
* tests/core/mempool.cc: test the new allocator