some more refactoring

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

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

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

src/test/scala/leon/test/purescala/TreeOpsTests.scala

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