Move Wasmtime for .NET to the Wasmtime repo.

This moves the Wasmtime for .NET implementation to the Wasmtime repo.

Wasmtime for .NET is a binding of the Wasmtime API for use in .NET.

crates/misc/dotnet/src/MutableGlobal.cs

@@ -0,0 +1,65 @@
+using System;
+
+namespace Wasmtime
+{
+    /// <summary>
+    /// Represents a mutable WebAssembly global value.
+    /// </summary>
+    public class MutableGlobal<T>
+    {
+        /// <summary>
+        /// Creates a new <see cref="MutableGlobal&lt;T&gt;" /> with the given initial value.
+        /// </summary>
+        /// <param name="initialValue">The initial value of the global.</param>
+        public MutableGlobal(T initialValue)
+        {
+            InitialValue = initialValue;
+            Kind = Interop.ToValueKind(typeof(T));
+        }
+
+        /// <summary>
+        /// The value of the global.
+        /// </summary>
+        public T Value
+        {
+            get
+            {
+                if (Handle is null)
+                {
+                    throw new InvalidOperationException("The global cannot be used before it is instantiated.");
+                }
+
+                unsafe
+                {
+                    var v = stackalloc Interop.wasm_val_t[1];
+
+                    Interop.wasm_global_get(Handle.DangerousGetHandle(), v);
+
+                    // TODO: figure out a way that doesn't box the value
+                    return (T)Interop.ToObject(v);
+                }
+            }
+            set
+            {
+                if (Handle is null)
+                {
+                    throw new InvalidOperationException("The global cannot be used before it is instantiated.");
+                }
+
+                // TODO: figure out a way that doesn't box the value
+                var v = Interop.ToValue(value, Kind);
+
+                unsafe
+                {
+                    Interop.wasm_global_set(Handle.DangerousGetHandle(), &v);
+                }
+            }
+        }
+
+        internal ValueKind Kind { get; private set; }
+
+        internal Interop.GlobalHandle Handle { get; set; }
+
+        internal T InitialValue { get; private set; }
+    }
+}