Update srml-contract to Rust 2018. (#1510)

* Fix imports.

* Remove redundant binding

* Clean extern crates.

* Add comment

* Re-export macros from prelude

* Build fixes

* Update core/sr-std/src/lib.rs

Co-Authored-By: pepyakin <s.pepyakin@gmail.com>

substrate/core/sr-std/src/lib.rs

@@ -45,4 +45,9 @@ pub mod prelude {
 	pub use ::boxed::Box;
 	pub use ::cmp::{Eq, PartialEq};
 	pub use ::clone::Clone;
+
+	// Re-export `vec!` macro here, but not in `std` mode, since
+	// std's prelude already brings `vec!` into the scope.
+	#[cfg(not(feature = "std"))]
+	pub use ::vec;
 }