rust update proposal

[Thomas Klausner <wiz%NetBSD.org@localhost>]

Hi!

We're still at rust 1.76, and the current version is 1.79.0.

As I understand the problem, some of the previously supported NetBSD
versions are not supported in 1.77+ any longer.

I'd like to have a newer rust in pkgsrc because some software already
requires it.

My proposal:

- Import (most of) lang/rust as lang/rust176 (to follow the existing
  naming convention from wip)

- Update lang/rust to 1.79

- In lang/rust/buildlink3.mk, add per-platform inclusion logic for
  using lang/rust176 for those platforms that are not working with
  1.79 (perhaps checking for RUST_REQ<=1.76 before doing that, and
  setting BROKEN (or one of the other variables for this) if not).

- Make sure that packages requiring 1.77+ set RUST_REQ correctly so
  that we save on bulk build cycles.

What do you think?

(When someone fixes the platform support, we can roll back part of
these changes and/or remove rust176.)
 Thomas