Initial draft for `use path as id;` RFC.
added revision to extern crate syntax.
Add kimundi's suggestion of `extern crate <ident> as <ident>` variant.

Note that I only put it in the Alternatives.  I think there is a
pretty good argument that whether to do this or not is orthogonal to
this RFC, and the only reason to fold it into this RFC is to avoid the
process overhead of filing a seprate RFC.
Merge remote-tracking branch 'pnkfelix/fsk-use-path-as-id'
"Use path as id" is RFC 47