Add source reader and template repr.

[ianjosephwilson]

• Adds SourceReader class.
  • Created when an exception occurs to read back the source for error messages.
• Adds MutableLinePosition class.
  • Accumulates the line position (w/ newlines) through interpolation expressions before showing the line/offset in error messages.
• Adds interpolation_repr(), template_repr_iter() and template_repr() functions.
  • Utility functions for helping SourceReader.

This code is sort of disconnected until we add in the actual exception/error handling.

I was able to sub out the slicing concept for the TemplateSpan().extract() pretty easily but other parts might change ref for span but I'm not sure until we add the next chunk.

[ianjosephwilson]

@davepeck Almost to the parser exceptions. It is amazing how much code and "shoehorning" all these details take.

[davepeck]

Awesome, thanks @ianjosephwilson!

Some comments here to consider but overall a nice narrow slice.

PS: I merged in #162 and updated the tests in this PR to use the new coordinates.

[davepeck]

@ianjosephwilson Looks good to me. LMK if you're ready for a merge. Thanks!

[ianjosephwilson]

@davepeck Should be ready to merge. I'll go back over the coverage at the end because some of these small methods we might just remove.