* adds `sized_range` and conformance tests
* moves `disable_sized_range` into namespace `std::ranges`
* removes explicit type parameter
Implements part of P0896 'The One Ranges Proposal'.
Differential Revision: https://reviews.llvm.org/D102434
Before this commit, we'd get a compilation error because the operator() overload was ambiguous.
Differential Revision: https://reviews.llvm.org/D102263