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
     }
 

Tests/SWBUtilPerfTests/PathPerfTests.swift

@@ -10,6 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+import Foundation
 import Testing
 import SWBTestSupport
 import SWBUtil
@@ -142,4 +143,19 @@ fileprivate struct PathPerfTests: PerfTests {
             #expect(lengths == (path.str.utf8.count + 1 + path2.str.utf8.count) &* N)
         }
     }
+
+    @Test
+    func bridgedPerf_X100000() async {
+        let string = (String(repeating: "/hello/little/world", count: 10) as NSString).standardizingPath
+        let path = Path(string)
+        let N = 100_000
+        await measure {
+            var lengths = 0
+            for _ in 0..<N {
+                let result = path.normalize()
+                lengths = lengths &+ result.str.utf8.count
+            }
+            #expect(lengths == string.utf8.count &* N)
+        }
+    }
 }