feat: optimise format phone number

Author: wmundev

csharp/PhoneNumbers.PerformanceTest/README.md

@@ -10,3 +10,7 @@ Below you can see a sample of what the results might look like
 | FormatPhoneNumber | .NET 8.0           | .NET 8.0           | 1.141 us | 0.0080 us | 0.0071 us | 0.0076 |     152 B |
 | FormatPhoneNumber | .NET Framework 4.8 | .NET Framework 4.8 | 2.746 us | 0.0114 us | 0.0101 us | 0.1335 |     851 B |
 
+## Running performance tests
+```bash
+dotnet run -c Release --framework net8.0
+```

csharp/PhoneNumbers/PhoneNumberUtil.net.cs

@@ -687,11 +687,8 @@ private void Format(ref Span<char> span, ref int index, PhoneNumber number, Phon
                 // Early exit for E164 case since no formatting of the national number needs to be applied.
                 // Extensions are not formatted.
                 PrefixNumberWithCountryCallingCode(ref span, ref index, countryCallingCode, PhoneNumberFormat.E164);
-                for (var i = 0; i < nationalSignificantNumberLength; i++)
-                {
-                    span[index++] = nationalSignificantNumber[i];
-                }
-
+                nationalSignificantNumber.Slice(0, nationalSignificantNumberLength).CopyTo(span.Slice(index));
+                index += nationalSignificantNumberLength;
                 return;
             }
 
@@ -701,11 +698,8 @@ private void Format(ref Span<char> span, ref int index, PhoneNumber number, Phon
             var regionCode = GetRegionCodeForCountryCode(countryCallingCode);
             if (!HasValidCountryCallingCode(countryCallingCode))
             {
-                for (var i = 0; i < nationalSignificantNumberLength; i++)
-                {
-                    span[index++] = nationalSignificantNumber[i];
-                }
-
+                nationalSignificantNumber.Slice(0, nationalSignificantNumberLength).CopyTo(span.Slice(index));
+                index += nationalSignificantNumberLength;
                 return;
             }