semilinear_util: minor refactoring
simplifiers: make IsActive() static
semilinear_set: fix a bug in the simplifier
sparse_vec: minor improvements
small improvements in a few places
simplifiers: implement a more flexible interface
var: inline operators
var: rework variable identifiers
sparse_vec: use emplace instead of []
tests: follow changes to Var
pseudo_linear: minor updates
polynomial: better type for Map
semilinear: huge overhaul of the semilinear sets

This implements ability to use a Divider for a SparseVec and exposes
more flexibility of specifying how the (Semi)LinearSet should be
simplified.
main: make it possible to use PseudoLinearSet
pseudo: remove old comment
Merge pull request #25 from michalt/semilinear

Semilinear improvements