Benchmark macro (#4962)

* MAcro benchamrks

* Iterative macro

* Tidying it up.

* Macro improvements

* Bits..

* Last benchmaks.

* Repo benchmark macro

* Add the possibility of evaluating arbitrary expressions in a
benchmaark

* Better syntax and docs

* Update `BenchmarkParameter`

* Add `ignore` to sudo-code in docs

* First try of timestamp implementation.

* Fix macro docs, remove warnings.

* Use macro in balances pallet.

* Make some space in frame benchmarking.

* Remove _benchmarks_seed variable.

* Bump impl_version.

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>
Co-authored-by: Marcio Diaz <marcio@parity.io>