lang/rust: Update to latest stable version (1.2.0)

[andoriyu_gmail.com]

Summary

Pretty straight forward change:

• Updated distfiles information (rust source and stage0)
• Adjusted pkg-plist

[dumbbell]

I think D3234 should be committed first or merged with this patch; in either case, this patch will need changes.

[jbeich]

Likely superseded by rP395967. If there's anything left to salvage better open new differential.

[andoriyu_gmail.com]

In D3490#73596, @jbeich wrote:

Likely superseded by rP395967. If there's anything left to salvage better open new differential.

nope, nothing to salvage. I do have another diff for D3491, will submit it in a bit.