Create benchmarks for ImmutableSet

[gabbard]

These could probably be based on the Guava benchmarks: https://github.com/google/guava/tree/master/guava-tests/benchmark/com/google/common/collect

[gabbard]

Quick things to try:

1. (from @ConstantineLignos ) try using a dict with None values instead of a frozenset as backing
2. Is it faster if we use a regular set instead of a frozenset as backing?

[gabbard]

@nicomitchell : additional benchmarks to add:

• item containment checks (both present and absent)
• iteration
• intersection, union, and difference
• all of the above with empty sets

[gabbard]

@ConstantineLignos : Please add any other requested benchmarks

[ConstantineLignos]

Nothing to add right now, sounds like good coverage.

[nicomitchell]

• benchmark for if item == 0 or item == 1 or if item in (0, 1) in singleton immutable sets (see #25 )