perf: stackalloc single char span, instead of using ToString, for .NET 7

This removes the allocations caused by ToString(). Unfortunately, the ReadOnlySpan<char> overload is only available from .NET 7, and so this is a conditional performance gain. Shame.

CHANGELOG.md

@@ -38,6 +38,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 BigEndian/LittleEndian methods.
 - X10D: `Stream.GetHash<>` and `Stream.TryWriteHash<>` now throw ArgumentException in lieu of
 TypeInitializationException.
+- X10D: `char.IsEmoji` no longer allocates for .NET 7.
 
 ### Removed
 

X10D/src/Text/CharExtensions.cs

@@ -18,7 +18,13 @@ public static class CharExtensions
     [MethodImpl(CompilerResources.MethodImplOptions)]
     public static bool IsEmoji(this char value)
     {
-        return value.ToString().IsEmoji();
+#if NET7_0_OR_GREATER
+        Span<char> chars = stackalloc char[1];
+        chars[0] = value;
+        return EmojiRegex.Value.IsMatch(chars);
+#else
+        return EmojiRegex.Value.IsMatch(value.ToString());
+#endif
     }
 
     /// <summary>