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/IO/DecimalExtensions.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics;
 using System.Diagnostics.Contracts;
 using System.Runtime.InteropServices;
 
@@ -93,7 +94,7 @@ public static class DecimalExtensions
     private static void GetBits(decimal value, Span<int> destination)
     {
 #if NET5_0_OR_GREATER
-        decimal.GetBits(value, destination);
+        _ = decimal.GetBits(value, destination);
 #else
         Span<byte> buffer = stackalloc byte[16];
         MemoryMarshal.Write(buffer, ref value);
@@ -101,6 +102,7 @@ public static class DecimalExtensions
 #endif
     }
 
+    [Conditional("NET5_0_OR_GREATER")]
     private static void WriteBits(Span<int> destination, Span<byte> buffer)
     {
         var flags = MemoryMarshal.Read<int>(buffer[..4]);

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>