## Summary
Previously we held off from doing this because we weren't sure that it
was worth the added complexity cost. But our code has changed in the
months since we made that initial decision, and I think the structure of
the code is such that it no longer really leads to much added complexity
to add precise inference when unpacking a string literal or a bytes
literal.
The improved inference we gain from this has real benefits to users (see
the mypy_primer report), and this PR doesn't appear to have a
performance impact.
## Test plan
mdtests
This PR updates our iterator protocol machinery to return a tuple spec
describing the elements that are returned, instead of a type. That
allows us to track heterogeneous iterators more precisely, and
consolidates the logic in unpacking and splatting, which are the two
places where we can take advantage of that more precise information.
(Other iterator consumers, like `for` loops, have to collapse the
iterated elements down to a single type regardless, and we provide a new
helper method on `TupleSpec` to perform that summarization.)
