Basic RSpec/benchmark-ips powered benchmark suite

Corresponding issue: #2909, see the commit messages for more details.

A few things to note:

1. The current use of `subject` isn't exactly easy on the eyes due to them having to return a Proc, I'm not sure yet how (and if) we can work around this.
2. The maximum amount of iterations in the current `User.by_login` benchmark is arbitrary, we might have to adjust it once said method's performance has been improved.
3. Benchmarks currently take 2 seconds to warm up and 5 seconds to run (benchmark-ips defaults). 
4. The custom RSpec matcher file (`benchmark_matchers.rb`) is a bit messy, any feedback on this would be appreciated

Any comments/feedback on this would be greatly appreciated.

See merge request !1503