Fix DocOpt deserialization type bounds
Fix DocOpt deserialization type bounds

This is wrt https://github.com/docopt/docopt.rs/pull/222

DocOpt does not support deserializing borrowed types.

This change was reverted in
https://github.com/docopt/docopt.rs/commit/7292a374e69afb192bb7aaa00f9d9f4afebc200d
because it broke crates like Cargo etc.

nipunn1313/cargo,nipunn1313/cargo,eminence/cargo,tsurai/cargo,tshepang/cargo,eminence/cargo,rust-lang/cargo,tsurai/cargo,rust-lang/cargo,rustyhorde/cargo,tsurai/cargo,tsurai/cargo,rust-lang/cargo,nipunn1313/cargo,tshepang/cargo,eminence/cargo,tsurai/cargo,eminence/cargo,vojtechkral/cargo,rustyhorde/cargo,rust-lang/cargo,vojtechkral/cargo,vojtechkral/cargo,nipunn1313/cargo,rustyhorde/cargo,vojtechkral/cargo,tshepang/cargo,tshepang/cargo,rustyhorde/cargo