Adds a `ProcMacro` form of syntax extension#36154

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bors merged 2 commits intorust-lang:masterfrom

nrc:proc-macro-init

Sep 23, 2016

Commits on Sep 21, 2016

Adds a `ProcMacro` form of syntax extension
nrc
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Commits on Sep 22, 2016

reviewer comments and rebasing
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