Simple incremental code generation

JastAdd2 could be a bit lazier regarding Java code generation. If the Java source files are newer than the corresponding aspect files that affect them JastAdd2 could skip re-generating the Java code.

Imagine an aspect A which adds things to class X. If those things are simple inter-type-declarations (synthesized attributes, methods or fields) then only X.java should need to be re-generated if only A was changed since last time the AST was generated.