[MacPorts] #40616: rust: update to 0.8

[MacPorts]

#40616: rust: update to 0.8
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 Reporter:  macports.org@…  |      Owner:  macports-tickets@…
     Type:  update          |     Status:  new
 Priority:  Normal          |  Milestone:
Component:  ports           |    Version:  2.2.0
 Keywords:                  |       Port:  rust
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 https://mail.mozilla.org/pipermail/rust-dev/2013-September/005804.html

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Ticket URL: <https://trac.macports.org/ticket/40616>
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