* Emit a PaymentParameters event once per block

This contains per-block paramaters need to calculate
fees off-chain.

* Add WeightToFee trait

* Add documentation to polynomial types

* Ignore pseudo code snippet for doc tests

* Use `Mul` implementation of Perbill

* Add tests for WeightToFeePolynomial

* Revert "Emit a PaymentParameters event once per block"

This reverts commit 6c4763ba

.

Co-authored-by: Gavin Wood <[email protected]>