now functional. Added map_builder classes for imperatively
construction maps. Initial collection of map implementations
(explicit, zero, 1, and composition.)
(sseq, module) appropriately; removed algebra/laurentpoly.h and added
algebra/multivariate_laurentpoly.h. Added io, muladdeq to Z. Added
functions open_file, close_file to lib/io.h. Added operator < to
map_wrapper.