rust-robust 1.2.0-2 is ready for sponsorship to unstable.

This promotes the existing experimental package to unstable so it can satisfy
the spade dependency chain needed by raygeo.

Build status:
 * ./build.sh robust completed successfully on amd64.
 * Raw autopkgtest summary is PASS for rust-robust:@,
   librust-robust-dev:default, librust-robust-dev:no_std, and
   librust-robust-dev:.
 * debcargo-conf reported UNKNOWN while analyzing the autopkgtest log, but the
   raw log says "Autopkgtest: pass" and "Status: successful".

Lintian:
 * lintian --display-info --pedantic was run on
   rust-robust_1.2.0-2_amd64.changes.
 * The package has the usual Rust crate informational/pedantic tags for files
   shipped under /usr/share/cargo/registry and old debhelper compat 13.
