Use optimized append and len storage methods in SRML. (#3071)

* expose len from codec to storage.

* refactor runtime with len and append.

* Undo example.

* Remove imports.

* Bump codec.

* Optionify.

* Make decode_len counscious.

* Refactor.

* Update srml/support/src/storage/hashed/generator.rs

Co-Authored-By: Bastian Köcher <[email protected]>

* Update srml/support/src/storage/hashed/generator.rs

Co-Authored-By: Bastian Köcher <[email protected]>

* Update srml/support/src/storage/hashed/generator.rs

Co-Authored-By: Bastian Köcher <[email protected]>

* Update srml/support/src/storage/hashed/generator.rs

Co-Authored-By: Bastian Köcher <[email protected]>

* Fix merge.

* fix some docs.

* Add NoDefault trait.

* Bump.

* Final nits.

* Update srml/support/src/traits.rs

* new approach toward len.

* re-create lock file.

* Fix build errors and Option handling.

* More test fix

* Use default for append as well.

* Fix runtime.

* Add support for linked_map

* More tweaks from review.

* Fix style

* Change api for none-values

* Bump.