Performance penalty for Log.debug

Kitura's detailed logging is applied sparingly at present (we don't do much logging), because there is a small overhead to a log statement. Having many of these on the critical path hurts performance measurably.

Even though log statements wrap the message String in an `@autoclosure` (#18), the arguments still have to get passed through to a call (that then typically does nothing). This includes default values of `#file`, `#function` as well as the creation of the closure itself.

There are a couple of things we could try in this area:
- We could provide alternate versions of 'expensive' log levels (verbose, debug, entry, exit) which can be used to disable those levels at compile time, for example:
```swift
#if PRODUCTION
public class func debug(_ msg: @autoclosure () -> String, functionName: String = "",
lineNum: Int = 0, fileName: String = "") {
return
}
#else
(existing implementation)
#endif
```
- Swift 4.2 introduces an [@inlinable](https://github.com/apple/swift-evolution/blob/master/proposals/0193-cross-module-inlining-and-specialization.md) attribute which would allow calls to an empty function (like the above) to be completely eliminated.