[mypyc] Add vec extend function to librt.vecs (#21340)
This is similar to the `append` function, but the second parameter is an
iterable instead of a single item.
This is fairly straightforward, but we need multiple implementations for
the different vec representations. We also have separate functions for
`extend` with two vec arguments, and for `extend` with any iterable as
the second argument.
Also support the `extend(v, v)` special case.
We can later add optimized support for the buffer protocol (e.g. extend
`vec[u8]` with a `bytes` object).
