Update for Rust syntax changes

extern crate foo = "bar";
is now
extern crate "bar" as foo;
as per RFC 47.  Details at rust-lang/rust#16461

src/rust-crypto-util/tool.rs

@@ -8,7 +8,7 @@
 #![crate_name = "rust-crypto-util"]
 
 extern crate getopts;
-extern crate rust_crypto = "rust-crypto";
+extern crate "rust-crypto" as rust_crypto;
 
 use std::io;
 use std::os;