[stdlib] Resolving some FIXME comments on Set type. (#20631)
* Fixing some fixmes on stdlib Set
* Adding @inline attr
* Fixing spaces
* Adding isEmpty as fast path in other places where is possible.
* Quotes on variable name on comment.
* Update stdlib/public/core/Set.swift
Co-Authored-By: LucianoPAlmeida <passos.luciano@outlook.com>
* Adding benchmark for isDisjoint Set method
* Adding empty sets to benchmark
* Fixing the factor on benchmarks and naming warnings for empty 5 words
