fixed hashCode for SymbolicConstraint; refactored hashCode caching
fixed bug introduced in previous commit by caching the hash code for SymbolicConstraint and SymbolicConstraint.Equality
added copyright
made SymbolicConstraint#Equality immutable
more fixes to SymbolicConstraint#Equality
fixed copyright
Merge pull request #349 from andreistefanescu/master

fixed hashCode for SymbolicConstraint; refactored hashCode caching