Refactor decl storage (#3765)

* split implementation in multiple files:
  * transformation -> genesis_config/  getters.rs  instance_trait.rs  metadata.rs  mod.rs  store_trait.rs
  * mod.rs -> parser.rs
  * impl.rs -> storage_struct.rs
* parser is isolated into parse module, it could be improved as well but this can be done in another PR
* modules contains a defintion of decl_storage input which must be ok to work with.
* implementation change:
  * T: Trait might be more often bound to 'static (anyway we only use static one and it is needed for metadata current implementation).
  * GenesisConfig no longer requires its fields to be Clone (possible since to EncodeLike feature)
  * builder for map storages must return precise type Vec<(key, value)>