refactor ArithmeticNormalization into TreeNormalizations

src/main/scala/leon/purescala/TreeNormalizations.scala

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

src/main/scala/leon/synthesis/rules/IntegerEquation.scala

...main/scala/leon/synthesis/rules/IntegerInequalities.scala

...t/scala/leon/test/purescala/TreeNormalizationsTests.scala