Clean up Uri.UnescapeDataString (dotnet/corefx#42225)

- Use string.IndexOf rather than an open-coded, unsafe loop.
- Avoid an unnecessary SequenceEquals at the end: we're only at this point if a `%` was found highlighting that something escaped was found.
- Use stack memory for smaller inputs if possible, to avoid unnecessary ArrayPool interaction
- Remove an unnecessary argument to a helper function.
- Fix ValueStringBuilder.Grow to only copy the contained data.

Signed-off-by: dotnet-bot <dotnet-bot@microsoft.com>

src/System.Private.CoreLib/shared/System/Text/ValueStringBuilder.cs

@@ -286,7 +286,7 @@ private void Grow(int additionalCapacityBeyondPos)
 
             char[] poolArray = ArrayPool<char>.Shared.Rent(Math.Max(_pos + additionalCapacityBeyondPos, _chars.Length * 2));
 
-            _chars.CopyTo(poolArray);
+            _chars.Slice(0, _pos).CopyTo(poolArray);
 
             char[]? toReturn = _arrayToReturnToPool;
             _chars = _arrayToReturnToPool = poolArray;