no more index argument

[alexreardon]

Closes #39
Closes #47
Rolls back #42

Rather than adding more arguments to our custom equality function: we provide examples on how to create higher order functions for passing in extra details to equality functions.

// recently added index in 4.1
- (a: mixed, b: mixed, index: number) => boolean
// removing it for compatibility reasons
+ (a: mixed, b: mixed) => boolean

This will be a breaking change as it will rollback the index argument

[alexreardon]

some extra tests to assert argument checking is opted out

[alexreardon]

Thoughts @ehtb?

[alexreardon]

/cc @treshugart

[alexreardon]

As a part of this I will be releasing 5.0 and deprecating 4.1