Add support for passing LocalDefs through chains.

So that it becomes possible to inline a closure used two
levels of inlining (or more) from where it is declared.
Handle This() more like VarRef() in the optimizer.
Inline methods of IndexedSeqOptimized in an ad hoc way.

Just because.
Add New to cases of SimpleMethodBody.
Inline more simple methods.
Discard VarDefs that are not actually used.
Discard side-effect-free LoadModules if not used.