Ensure Path _str takes UTF-8 String fast paths

Some clients (*cough* *cough* Xcode) like to create Paths out of bridged
NSStrings, which cannot be efficiently iterated. As most of our
operations require doing this frequently, we might as well convert on
construction to avoid the -characterAtIndex: slow path.

Sources/SWBUtil/Path.swift

@@ -104,6 +104,8 @@ public struct Path: Serializable, Sendable {
     }
 
     public init(_ str: String) {
+        var str = str
+        str.makeContiguousUTF8()
         self._str = str
     }