Investigate performance regression in fromJS()

[Methuselah96]

See #210 for why this issue exists.
See #211 for the code used in the benchmark.

Note Lee's thoughts when someone proposed our current solution:

• fromJS() only works on Objects in the same Realm immutable-js/immutable-js#1039
• Immutable.fromJS bug with no classes objects with custom constructor property immutable-js/immutable-js#1056

Thought so far:

• Is this the most performant way to get the correct behavior? If so, do we just go with it? Or do we just not support objects in separate realms?
• What other options are available to us?

[Methuselah96]

At the end of the day, I'm not confident in the way that we are measuring performance (even with the same code, I've often seen benchmark differences of 10% and more). I am working on a more comprehensive strategy to determine what our overall performance goals are.

@bdurrer re-ran some of the benchmarks with some more test cases outside of a VM context with slightly less worrying results (see screenshot below). Since we don't have a comprehensive plan of what performance levels are acceptable, I'm closing this for now (with a new issue to comes soon to discuss strategies for determining what our performance goals are: #214).