Improve compatibility of Python and Lean DDM implementation

[joehendrix]

This is a large PR that makes DDM changes to support source location information in #112.

• Updates SyntaxCat to use a structure instead of algebraic data type. This improves efficiency of accessing head.
• ArgDecls is a singleton structure instead of type abbreviation and coercisions with array no longer allowed. This will allow additional invariants to be added such as uniqueness of names.
• There is additional checking for undefined names to reduce spurious errors when module imports cannot be found.

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[aqjune-aws]

It seems the files in Examples/Lean have a few broken examples - should we fix these?

[aqjune-aws]

I could confirm that the BoogieLite example is now fixed. :)