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/procedural/tools/src/syn_ext.rs

@@ -31,7 +31,7 @@ use syn::Ident;
 
 /// stop parsing here getting remaining token as content
 /// Warn duplicate stream (part of)
-#[derive(ParseStruct, ToTokensStruct, Debug)]
+#[derive(Parse, ToTokens, Debug)]
 pub struct StopParse {
 	pub inner: T2,
 }