allow slurping in any position (#42902)
This extends the current slurping syntax by allowing the slurping to not
only occur at the end, but anywhere on the lhs. This allows syntax like
`a, b..., c = x` to work as expected.
The feature is implemented using a new function called `split_rest`
(definitely open to better names), which takes as arguments the
iterator, the number of trailing variables at the end as a `Val` and
possibly a previous iteration state. It then spits out a vector
containing all slurped arguments and a tuple with the n values that get
assigned to the rest of the variables. The plan would be to customize
this for different finite collection, so that the first argument won't
always be a vector, but that has not been implemented yet.
`split_rest` differs from `rest` of course in that it always needs to be
eager, since the trailing values need to be known immediately. This is
why the slurped part has to be a vector for most iterables, instead of a
lazy iterator as is the case for `rest`.
