Remove unsafe code and make scheme parsing faster in Uri (#90087)

* Remove unsafe code and make scheme parsing faster in Uri

* Reduce the number of range checks

* Switch loop to IndexOf

src/libraries/System.Private.Uri/src/System/DomainNameHelper.cs

@@ -77,11 +77,7 @@ internal static string ParseCanonicalName(string str, int start, int end, ref bo
             if (index >= 0)
             {
                 // We saw uppercase letters. Avoid allocating both the substring and the lower-cased variant.
-                return string.Create(end - start, (str, start), static (buffer, state) =>
-                {
-                    int newLength = state.str.AsSpan(state.start, buffer.Length).ToLowerInvariant(buffer);
-                    Debug.Assert(newLength == buffer.Length);
-                });
+                return UriHelper.SpanToLowerInvariantString(str.AsSpan(start, end - start));
             }
 
             if (str.AsSpan(start, end - start) is Localhost or Loopback)

src/libraries/System.Private.Uri/src/System/UriHelper.cs

@@ -10,6 +10,17 @@ namespace System
 {
     internal static class UriHelper
     {
+        public static unsafe string SpanToLowerInvariantString(ReadOnlySpan<char> span)
+        {
+#pragma warning disable CS8500 // takes address of managed type
+            return string.Create(span.Length, (IntPtr)(&span), static (buffer, spanPtr) =>
+            {
+                int charsWritten = (*(ReadOnlySpan<char>*)spanPtr).ToLowerInvariant(buffer);
+                Debug.Assert(charsWritten == buffer.Length);
+            });
+#pragma warning restore CS8500
+        }
+
         // http://host/Path/Path/File?Query is the base of
         //      - http://host/Path/Path/File/ ...    (those "File" words may be different in semantic but anyway)
         //      - http://host/Path/Path/#Fragment