Use an out-of-tree unsafe-any trait instead of a home-grown version.

This has the advantage of being separately checked, tested
and documented.

Cargo.toml

@@ -10,8 +10,12 @@ git = "https://github.com/servo/rust-url"
 [dependencies.mime]
 git = "https://github.com/seanmonstar/mime.rs"
 
+[dependencies.unsafe-any]
+git = "https://github.com/reem/rust-unsafe-any"
+
 [dev-dependencies.curl]
 git = "https://github.com/carllerche/curl-rust"
 
 [dev-dependencies.http]
 git = "https://github.com/chris-morgan/rust-http"
+