Exception.stracktraces now format anonymous functions more readably.

I always found the notation for anonymous functions in stacktraces very confusing. 

This PR tries to reformat them into something more approachable:

```
** (RuntimeError) BOOM!
    test.ex:3: Outer.func/2
    test.ex:11: second anonymous fn/1 in Inner.other/3     #<<<<<< 
    test.ex:12: Inner.other/3
    (elixir) src/elixir_lexical.erl:17: :elixir_lexical.run/2
    (elixir) lib/code.ex:301: Code.require_file/2
```