Change inlining to use an approach similar to the one of SimpleLanguage

- keep copy of uninitialized method AST
- do not use `cloneForInlining()` any longer
- replace `methodCloneForInlining()` with `inline(..)`
- Method defines now the variants for the inlined methods of n-ary messages

Signed-off-by: Stefan Marr <git@stefan-marr.de>