CircularBuffer: Add benchmarks and improve performance

[vyu]

This PR adds a set of benchmarks for CircularBuffer and also improves its performance.

Here are some PkgBenchmark reports using the benchmarks and comparing the new performance:

• Comparison with baseline on Julia 1.4.
• Comparison with baseline on Julia 1.0.
• Timings on Julia 1.4, on an N2D instance on GCP.

The benchmark file can be called directly by PkgBenchmark using the script keyword. For example, PkgBenchmark.benchmarkpkg("DataStructures"; script="benchmark/bench_circular_buffer.jl").

I've added review comments to explain the rationale behind some of the changes.

There was some inconsistency in the code over whether to use accessor functions (capacity(cb)) or dot notation (cb.capacity) to read field values. For consistency within the file, I've modified them to use accessor functions. They generate identical code since the functions get inlined.

I haven't worked on append! and fill! yet because improving those is more complicated. I might get to them in the future if this PR goes through.

Thanks!

[vyu]

Thanks! I've modified lines with range iteration to follow your suggestion.

[vyu]

I just noticed that setindex!(cb::CircularBuffer, item, i) returns cb. This has been the case since the beginning (when CircularBuffer was added in v0.4.3) and this PR retains that behavior. But per setindex! in Julia Base, it should really return item, not cb. I think this is a bug that ought to be fixed, but fixing it is potentially breaking if there is code out there that depends on this unconventional behavior. I can open a separate PR for it.