[mypyc] Update vec value struct to have direct pointer to items (#21420)
Now the second field of a `vec` struct representation points to the
first data item. Previously the second field of the `vec` struct
representation was a `PyObject *` targeting the buffer object. This
removes one pointer addition per get item or set item operation. Since
these are often the most performance critical operations, we want them
to be as efficient as possible. This should reduce the number of
instructions at least on ARM, and it will reduce code size on x64 as
well.
Since vec is still behind a feature flag, we can still break the ABI.
Used heavy coding agent assist, but iterated until the results look
good.
mypy
e7846d96 - [mypyc] Update vec value struct to have direct pointer to items (#21420)
