Add pySpec generator

[joehendrix]

Description of changes:

This PR is mostly about adding a pySpec generator, but does some other cleanups that will hopefully be uncontroversial.

The "pySpec generator" is a prototype strata subcommand that takes a Python file as an argument, and produces a PySpec DDM Program for the file and its imports. The Python files it processes are intended to capture Python method signatures and their pre and post-conditions as idiomatic Python source code. This should facilitate use of LLMs to generate them.

PySpec files are binary files that have semi-compiled the specifications. This is intended to make them efficient to load PySpec files when trying to validate source code that uses those imports.

The minor changes include:

1. Migrating some of the source mapping information from the DDM AST package to a Utils package to reduce unnecessary code in the DDM.
2. Fixing a minor bug in parsing DDM Ident types in Ion files.
3. Making the FormatState empty def public so it is public inhabited.

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