Replace JSON metadata with custom metadata structures (#766)

* Move module metadata from json string to custom metadata

* Revisit the metadata structures

1. Move the structures into the metadata crate.
2. Switch to using Cow/MaybeOwnedArray to support Encode/Decode

* Adapt to new metadata structures

* Convert event json metadata to new metadata structures

* Convert storage json metadata to new metadata structures

* Convert runtime metadata from json to new metadata structs

* Implements new metadata structures in client and runtime

* Fixes errors on `no_std`

* Fixes errors after rebasing master

* Do not use `Cow` anymore in metadata

Also replace `String` with our own type definition `StringBuf`.
This fixes compilation on `no_std`.

* Wrap `RuntimeMetadata` in `RuntimeMetadataVersioned` to support versioning

* Move metadata into `srml` and make core unaware of the implementation