Implement String <-> Span<char> conversions (#1090)

* Add Span.ToString()

(cherry picked from commit 8861109)

* Implement string <-> Span<char> conversions

(cherry picked from commit a54affc)

Source/Mosa.Korlib/System/Span.cs

@@ -137,4 +137,14 @@ public Span<T> Slice(int start, int length)
 
 		return new Span<T>(ref Unsafe.Add(ref _pointer.Value, (nint)(uint)start), length);
 	}
+
+	public override string ToString()
+	{
+		if (typeof(T) == typeof(char))
+		{
+			return new string(new ReadOnlySpan<char>(ref Unsafe.As<T, char>(ref this._pointer.Value), this._length));
+		}
+
+		return $"{nameof(System)}.{nameof(Span<T>)}";
+	}
 }

Source/Mosa.Korlib/System/String.cs

@@ -8,6 +8,7 @@
 using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using System.Text;
+using Stack = System.Collections.Stack;
 
 namespace System;
 
@@ -340,6 +341,17 @@ public override bool Equals(object obj)
 		return !Equals(a, b);
 	}
 
+	public static unsafe implicit operator ReadOnlySpan<char>(string s)
+	{
+		if (s == null) return ReadOnlySpan<char>.Empty;
+		return new ReadOnlySpan<char>(s.first_char, s.length);
+	}
+
+	public static implicit operator string(Span<char> span)
+	{
+		return span.ToString();
+	}
+
 	public static unsafe bool Equals(string a, string b)
 	{
 		if (a == null || b == null) { return false; }