Add support for rust-peg output when working with slices

[garretfick]

Motivation for change

rust-peg trace statements are in one of two forms.

When the input is a str, then the statements are line/column:

[PEG_TRACE] Attempting to match rule `nums` at 1:1
[PEG_TRACE] Matched rule `nums` at 1:1 to 1:5

When the input is a slice, then the statements are indices:

[PEG_TRACE] Attempting to match rule `list` at 0
[PEG_TRACE] Matched rule `list` at 0 to 5`

pegviz failed when parsing the index form because it expected only the line/column form.

What changed

This pull request adds support for the index form. There are two parts to the proposed change:

• update the parser to support both forms
• change the Location struct to an enum to support both forms
• change the pos function to support either Location form

How do I know it works?

I wrote two integration tests that parse output that I generated from rust-peg. I also tested with output from ironplc (a project I'm building that uses rust-peg) using both the str and slice variations of the parser.