Avoid Paths with noncontiguous foreign strings

Path relies on fast iteration of its backing _str for most of its
operations. To avoid the performance hit of repeatedly calling
-characterAtIndex: when constructed with an unfortunate string, update
clients to avoid doing this.

Sources/SWBUtil/Path.swift

@@ -139,6 +139,11 @@ public struct Path: Serializable, Sendable {
 
     public static var homeDirectory: Path {
         var rawPath = NSHomeDirectory()
+        // NSHomeDirectory produces a NSPathStore2, which is not a contiguous
+        // UTF-8 layout. Most of Path's operations rely on efficient iteration
+        // of String, and we can dodge the -characterAtIndex: slow path by
+        // performing this conversion early.
+        rawPath.makeContiguousUTF8()
         #if os(Windows)
         if rawPath.hasPrefix("/") {
             rawPath.removeFirst()

Sources/SWBUtil/PropertyList.swift

@@ -658,7 +658,12 @@ private func convertToPropertyListItem(_ item: Any) -> PropertyListItem {
         // Expected these to fall into the NSNumber/CFNumber case (except on non-Darwin), but leaving this here in case that implicit conversion changes unexpectedly.
         return .plDouble(asDouble)
 
-    case let asString as String:
+    case var asString as String:
+        // It is likely that property list decoding has produced a string that
+        // is not contiguous UTF-8. While this doesn't break anything, it is a
+        // performance footgun for most clients. Eagerly convert it to a native
+        // representation to ensure they hit the String fast paths.
+        asString.makeContiguousUTF8()
         return .plString(asString)
 
     case let asData as Data: