Improvements and bug fixes to sllambda implementation

[FredrikTaquist]

• Changes the name of the rattt learner to sllambda
• Adds a new learner, sldt, for learning using discrimination trees, but taking suffixes directly from counterexamples
• Makes register mappings consistent between suffixes in the discrimination tree
• Improves equivalence testing to produce shorter test sequences
• Makes several improvements to suffix optimization of incremental suffixes used in discrimination tree
• Fixes various bugs in the implementation of the sllambda learning algorithm