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 <bkchr@users.noreply.github.com>

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

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

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

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

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

Co-Authored-By: Amar Singh <asinghchrony@protonmail.com>

* Update srml/example/src/lib.rs

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Final cleanup.

* Fix build.

substrate/core/consensus/rhd/src/lib.rs

@@ -32,6 +32,9 @@
 
 #![cfg(feature="rhd")]
 // FIXME #1020 doesn't compile
+// NOTE: this is the legacy constant used for transaction size. No longer used except
+// for the rhd code which is not updated. Placed here for compatibility.
+const MAX_TRANSACTIONS_SIZE: u32 = 4 * 1024 * 1024;
 
 use std::sync::Arc;
 use std::sync::atomic::{AtomicUsize, Ordering};