1. Turns out that there was no order-dependent
   issue in MyPy anymore; the issue was just
   that overloads had to be immediatel next to
   each other - interleaving the TypeVar lines
   with the overload lines was causing MyPy to
   stop seeing overloads as soon as the first
   line that contained something else was hit.
2. Putting the smaller-arity overloads first
   significantly speeds up MyPy checking in
   the typical (handful-of-arguments) cases,
   presumably because MyPy is checking them in
   declaration order (possibly due to Python's
   implementation of `typing.overload` rather
   than any decision by MyPy - I didn't look).