[Truffle] Reuse translateMethodDefinition in ModuleTranslator's def.
StringBuilder should delegate toString to the underlying implementation.

* Fixes output of bin/ast -s.
[Truffle] Fix splatting in ArgsCatNode and implement Hash#to_a.
[Truffle] Reuse ArrayGuards with @ImportGuards.
[Truffle] Fix handling of the rest argument with post arguments.

* Simplify too much indices computations by using negative indices.
* Introduce an ArraySliceNode.
[Truffle] Untag some new passing specs due to rest arg handling and better splat support.