Add some benchmark tests

[goodboy]

There's been some discussion (#15, #23) surrounding performance mostly as it pertains to the _Multicall.execute() loop.

I propose we add some tests which use pytest-benchmark or something similar to audit changes we make to critical code paths. This encourages being objective about future changes.

[goodboy]

Some other options to investigate:

• pytest-profiling
• the pytest benchmark scripts

[goodboy]

There was recently a post on the pytest mailing list about a new alternative bench-marking system as well.