Show method names and local variables in Truffle backtrace

@chrisseaton That should be a start!

Say we have `test.rb` with:
```ruby
def bar(b, c = 3.14)
  local = 42
  TruffleDebug.dump_call_stack
end

def foo(a)
  hidden = :hidden
  bar(2)
end

main = self
top = "level"*10
foo(1)
```
This outputs:
```
call stack ----------
    at TruffleDebug#dump_call_stack
    in TruffleDebug#dump_call_stack(core):TruffleDebug#dump_call_stack
	from test.rb:3 in #<Class:#<Object:0x71>>#bar with b = 2, c = 3.14, local = 42
	from test.rb:8 in #<Class:#<Object:0x71>>#foo with a = 1, hidden = :hidden
	from test.rb:13 in <main> with top = "levellevell... (String), main = #<Object:0x7... (Object)
---------------------
```
Not pretty yet, but it shows the possibilities.
BTW, the `#<Class:#<Object:0x71>>` reveals a bug in Truffle: methods defined at top-level should be defined on Object (and not main's singleton class) as private methods.