Include default values in storage metadata (#1264)

* Add a 'default' field to metadata. It contains code to generate the
default value.

* wasm update

* Make 'default' field an `Option`

* Boxed fn is not static, that won't be fine

* static fn won't do it to as it cannot get T param, will try fat trait

* Fat pointer over phantom data compatible with static instantiation

* DecodeDifferent is cool, using it for decoding.

* using once cell to do what would require copying lazy_static internals.

* Remove cache when no_std (non compatible deps)

* wasm bins update

* Fuse tooling struct and enum derive.

substrate/srml/support/src/lib.rs

@@ -45,6 +45,11 @@ extern crate parity_codec_derive;
 
 #[doc(hidden)]
 pub extern crate parity_codec as codec;
+
+#[cfg(feature = "std")]
+#[doc(hidden)]
+pub extern crate once_cell;
+
 pub use self::storage::generator::Storage as GenericStorage;
 
 #[macro_use]