* I believe these are also good mutations as lt vs lte and gt vs gte
differencies always will be observable from good specs and the
developer will always be very sure he uses the right one.
* Add Mutant::AST namespace to hold all AST related data / helpers.
* Mutant::AST will be externalized into an ast-meta gem that can be
shared with unparser for deduplication.
* Over the time the mutators itself will not need to deal with semantic
analysis of the AST anymore by themselves.
* Move AST analysis for send nodes to AST::Meta
* Fix#209
