Add reject_reason in MarketRejected event

[vivekvpandya]

This PR is intended to close #795

[sea212]

An update of docs/changelog_for_devs.md is also required.

[sea212]

What do you guys think about the range of r in the benchmarks? If we go up to 1024, then we have four million executions of this particular benchmark. I have no clue if that's too much.

This is fine @maltekliemann . The steps option in the benchmark command will limit the number of iterations significantly. Currently we use steps = 10 for Zeitgeist pallets, which results in r taking the values i * r_max/10, where i is the current iteration. Given the current configuration we have 4000 benchmark runs per extrinsic.