Add pre_check: :all_same option

[sabiwara]

Close #435

I tried to split individual commits to help with the review.

Note regarding b33b5cf

I noticed that Benchee.UnknownProfilerError might have accidentally been declared as Benchee.Profile.Benchee.UnknownProfilerError, hence the proposed move to a new file where we can define custom errors in the top-level Benchee. If nesting was voluntary here, I'm happy to replicate this pattern and define it within Benchee.Benchmark.Runner.

Note regarding the name

I'm OK with the originally proposed :assert_all_same as well, but since it is used within pre_check, the notion of checking/asserting felt slightly redundant and pre_check: :all_same felt enough. Happy to change it back or to something else entirely 🙂

Note regarding 44dff6f

I implemented a custom collector in order to retrieve the return value, but thanks to dialyzer I realized it didn't strictly speaking match the behaviour which expects a number. We could perhaps override scenario.function to a wrapper sending a message to the parent, but I wanted to consult you first about the best approach.