[Truffle] Add a method to dump the stack trace.
[Truffle] Add method name to backtrace output, MRI-style.
[Truffle] Show the values of local variables in the stack trace!

* RubyCallStack.dump() now takes a RubyContext.
* RubyNode.panic2() is removed as unused.
Merge pull request #1832 from eregon/truffle_backtrace

Show method names and local variables in Truffle backtrace