Remove temporary Vec usage in the http module (serenity-rs#2624, s…

…erenity-rs#2646)

This avoids having to allocate to store fixed length (replaced with normal
array) or fixed capacity (replaced with `ArrayVec`) collections as vectors for
the purposes of putting them through the `Request` plumbing.

Slight behavioral change - before, setting `params` to `Some(vec![])`
would still append a question mark to the end of the url. Now, we check
if the params array `is_empty` instead of `is_some`, so the question
mark won't be appended if the params list is empty.

Co-authored-by: Michael Krasnitski <[email protected]>