Move inherent implementation into the modules (#924)

* Adds new `ProvideInherent` trait

Also implements the new trait for `srml/consensus` and `srml/timestamp`.

* Adds `impl_outer_inherent!` macro

* Reexport macros from `alloc`

* Introduce `RuntimeString` and fix `ProvideInherent` on `no_std`

* Replace `VersionString` with `RuntimeString`

* Improvements for `impl_outer_inherent!`

* Make `construct_runtime!` support `impl_outer_inherent!`

* Fixes after rebase

* Whitespace

substrate/core/sr-std/without_std.rs

@@ -15,7 +15,8 @@
 // along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
 
 #[cfg(feature = "nightly")]
-extern crate alloc;
+#[doc(hidden)]
+pub extern crate alloc;
 
 extern "C" {
 	fn ext_malloc(size: usize) -> *mut u8;