refactor(snapbox): Changes to normalization

[Muscraft]

This is a part of #92.

This PR is to help move forward the effort with supporting structured data formats. Part of this effort showed that there needed to be a better way to specify what normalization operation was wanted and later to unify what the operations are doing.

• Abstract lower level map_text calls as normalize with operation types
  • By providing build blocks before, we were hoping to allow people to plug in their own normalization routines. However, when we add structured data support, it will need to operate on internal data initially as we work out details, so we're making all of the normalization logic private, just allowing the caller to which built-in normalization can be done.
• change as_str to try_string and update its return to be Option<String>
  • The original intent of as_str was for outputting Diffs. This had to be changed to String since there are some formats that have borrowing issues with &str. By changing to String it allows support for more formats