Basic RSpec/benchmark-ips powered benchmark suite

Corresponding issue: #2909 (closed), 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.