Per-transaction weight for srml (#2799)

* debug checkpoint.

* new

* Worked.

* Worked and weight propagated to executive.

* Works with some tests.

* Cleanup debug prints.

* More cleanup.

* Undo more logs.

* Undo a few more.

* Fix build.

* Allow len to be used in weight calculation.

* Remove noop function from dispath.

* Cleanup.

* Unify traits.

* Update docs and nits.

* line width

* Update core/sr-primitives/src/weights.rs

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

* Update core/sr-primitives/src/weights.rs

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

* Update core/sr-primitives/src/weights.rs

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

* Update core/sr-primitives/src/weights.rs

Co-Authored-By: Amar Singh <[email protected]>

* Update srml/example/src/lib.rs

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

* Final cleanup.

* Fix build.