[tk] Switch symbolic/shape placeholders to sympy

[stellaraccident]

By using sympy instead of the stand-in symbolic support, we get full support for index (partial) evaluation, dynamic dimensions and validation checks.

[stellaraccident]

@Groverkss This is passing the tests post my refactor to:

• rebase on sympy
• carry side-car index data along with the graph (so that we can do more analysis/checking at emission)

I dropped the SliceAnalysis thing and inlined it into some more direct cast utilities.

I'm going to keep iterating on this a little bit because now I can add the stuff at the top level to directly make symbols usable in the IR.

[Groverkss]

I like this a lot. I think I have some doubts about carrying the range analysis with the value and about slicing, but I think we can fix them later.

Let's land this and we can iterate on this further.

[stellaraccident]

Thanks. I don't like where that was going either. I needed it when I was in the ditch trying to make slice analysis robust... But then I just deleted it and went with something more direct. This revision still needs this for the moment for accessing dynamic grid values, but I don't like that and will scalpel it out.