Update lang/rust to 1.2.0 (issue with stage-qa)

[riggs]

Summary

Attached diff updates lang/rust to most recent stable.
There is however a catch: It insists to link to /lib/libedit.so.7
This apparently originates in the stage0 rustc which is used for
bootstrapping, so there seems no obvious way to avoid it. This causes
an error in stage-qa. A member of portmgr should review and advise.

[jbeich]

Likely superseded by rP395967. If there's anything left to salvage better open new differential.
And, I think, issues with no known fixes belong in bugzilla or on maillists, not here where the visibility is low.

[riggs]

Yes. Just wait a few weeks and it will become obsolete :-)