[SL.Core] Switch to ReadOnlySpan.

src/SixLabors.Core/Helpers/DebugGuard.cs

@@ -169,7 +169,7 @@ namespace SixLabors
         /// <paramref name="target"/> is true
         /// </exception>
         [Conditional("DEBUG")]
-        public static void MustBeSameSized<T>(Span<T> target, Span<T> other, string parameterName)
+        public static void MustBeSameSized<T>(ReadOnlySpan<T> target, ReadOnlySpan<T> other, string parameterName)
             where T : struct
         {
             if (target.Length != other.Length)
@@ -189,7 +189,7 @@ namespace SixLabors
         /// <paramref name="target"/> is true
         /// </exception>
         [Conditional("DEBUG")]
-        public static void MustBeSizedAtLeast<T>(Span<T> target, Span<T> minSpan, string parameterName)
+        public static void MustBeSizedAtLeast<T>(ReadOnlySpan<T> target, ReadOnlySpan<T> minSpan, string parameterName)
             where T : struct
         {
             if (target.Length < minSpan.Length)

src/SixLabors.Core/Helpers/Guard.cs

@@ -239,7 +239,7 @@ namespace SixLabors
         /// <exception cref="ArgumentException">
         /// <paramref name="target"/> is true
         /// </exception>
-        public static void MustBeSizedAtLeast<T>(Span<T> target, int minLength, string parameterName)
+        public static void MustBeSizedAtLeast<T>(ReadOnlySpan<T> target, int minLength, string parameterName)
         {
             if (target.Length < minLength)
             {