`DispatchQueue.sync` was required to avoid CoW on calling `subscript` from multiple copied instances of `Array`.
Since `UnsafeMutableBufferPointer` does not cause CoW on same situation, `DispatchQueue.sync` can be removed.
By applying concepts from Swift Talk 90: https://talk.objc.io/episodes/S01E90-concurrent-map
Notably:
* Removing `@escaping` from block parameter
* Avoiding Array.append & sorting
* Using a constant DispatchQueue label
also remove parallelForEach from SwiftLintFramework's public interface
since calling the GCD API is almost as concise and avoids polluting the
public API.
Finally, also remove the "fast" path for parallelMap since GCD should
do the reasonable thing for us here.
Norio Nomura
JP Simard
Marcelo Fabri
Michał Kałużny
Scott Hoyt