Support In-place Prettify

[reid-spencer]

Allow individual files to be prettified. This requires the definitional context to be provided as a parameter. That is, a top level fill can be included with "Root" level parsing while a file included into a "Domain" needs that level of context in order to know how to parse it.

With that support, it then becomes possible to:

• parse the input file correctly
• validate it by providing the top level context artificial
• write out a candidate replacement file
• parse & validate that replacement file
• remove the old file and rename the new files to have prettified a single file.