integrate equation synthesis as a rule

src/main/scala/leon/synthesis/ArithmeticNormalization.scala

src/main/scala/leon/synthesis/IntegerSynthesis.scala

src/main/scala/leon/synthesis/LinearEquations.scala

src/main/scala/leon/synthesis/Rules.scala

...la/leon/test/synthesis/ArithmeticNormalizationSuite.scala

...test/scala/leon/test/synthesis/LinearEquationsSuite.scala