[mlir][python] Fix issues with block argument slices

The type extraction helper function for block argument and op result
list objects was ignoring the slice entirely. So was the slice addition.
Both are caused by a misleading naming convention to implement slices
via CRTP. Make the convention more explicit and hide the helper
functions so users have harder time calling them directly.

Closes #56540.

Reviewed By: stellaraccident

Differential Revision: https://reviews.llvm.org/D130271