Merge pull request #19 from stevens-crag/SLPSet/recompression_performance_fix

Slp set/recompression performance fix
Fixed bug with a^2
Fixed a potential bug, but I think the fixed situation was impossible
Added support for negative powers
Half-working recompression reduce, before big refactoring the method of working with blocks
Refactored code so now no automatic collapsing of same consequent letters into block
Fixed recompression reduce && created profiling