Reintegrating optimizations originally proposed by @fy0 in issue: https://github.com/deckarep/golang-set/issues/60

[deckarep]

This branch is largely dedicated to an issue that got proposed that I lost track of during the refactor to Generics. Performance gains can be had with the smarter use of capacity hint upon instantiation the sets.

• Originally proposed by @fy0
• Utilize map capacity hint on make invocations as an optimization upon instantiation of Sets
• Fixes some inconsistencies around doc comments
• Additionally adds a RemoveAll method

Any takers on CRing this? I'll leave it open for a few days and merge if nobody bites.

Cheers

-deckarep