Make staking inflation curve configurable. (#3644)

* Draft for new design of NPoS rewards

* finish code

* fix test

* add tests

* improve log test

* version bump

* Update srml/staking/reward-curve/Cargo.toml

Co-Authored-By: Kian Paimani <[email protected]>

* u128 -> u64

* make conversion to smaller type safe

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

Co-Authored-By: Kian Paimani <[email protected]>