RFC: Added "Debug Info"

Because of the discussion in #466 I am proposing a more efficient way to freeze and inspect debug information.  This would allow an implementation to just keep track of the instruction pointer and later using `DWARF` information (or other) to locate the debug info.

[Rendered View](https://github.com/mitsuhiko/rfcs/blob/debug-info/text/0000-debug-info.md)