resort _ast and ast modules

[tungol]

Well, I got a lukewarm to positive reaction from https://github.com/python/typeshed/issues/11141, about what to do when c-implemented classes claim their module is the non-private one. So here's an MR to re-organize ast and _ast and get the type stubs to match what the implementation claims.

Mostly this is just lift and shift, but I took the opportunity to make the order within the file more closely match the implementation - that seems to be preferred?

related to https://github.com/python/typeshed/issues/3968 and the discussion in https://github.com/python/typeshed/issues/11141

[github-actions[bot]]

Diff from mypy_primer, showing the effect of this PR on open source code:

mypy (https://github.com/python/mypy)
+ mypy/fastparse.py:1612: error: Argument 1 to "visit" of "ASTConverter" has incompatible type "slice"; expected "Optional[AST]"  [arg-type]

[github-actions[bot]]

Diff from mypy_primer, showing the effect of this PR on open source code:

mypy (https://github.com/python/mypy)
+ mypy/fastparse.py:1612: error: Argument 1 to "visit" of "ASTConverter" has incompatible type "slice"; expected "Optional[AST]"  [arg-type]

[github-actions[bot]]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉