Swaps the `arrayvec` dependency for `heapless`, as the `heapless`
library allows changing the type used for the `len` field, which
`arrayvec` hard-codes to `usize`. This means that, for all the
`ArrayVec`s in Zed, we can save 7 bytes on 64 bit platforms by just
storing the length as a `u8`.

I have not benchmarked this change locally, as I don't know what
benchmarking tools are in this project.

As a small bit of context, I wrote the PR to `heapless` to add this
`LenT` generic after seeing a PR on the `arrayvec` crate that seems to
be dead now. Once I saw some of Zed's blog posts about the `rope` crate
and noticed the usage of `arrayvec`, I thought this might be a welcome
change.

Release Notes:

- N/A

---------

Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>