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/Externs/ExternGlobal.cs

@@ -0,0 +1,62 @@
+using System;
+using Wasmtime.Exports;
+
+namespace Wasmtime.Externs
+{
+    /// <summary>
+    /// Represents an external (instantiated) WebAssembly global.
+    /// </summary>
+    public class ExternGlobal
+    {
+        internal ExternGlobal(GlobalExport export, IntPtr global)
+        {
+            _export = export;
+            _global = global;
+        }
+
+        /// <summary>
+        /// The name of the WebAssembly global.
+        /// </summary>
+        public string Name => _export.Name;
+
+        /// <summary>
+        /// The kind of value for the global variable.
+        /// </summary>
+        public ValueKind Kind => _export.Kind;
+
+        /// <summary>
+        /// Determines whether or not the global variable is mutable.
+        /// </summary>
+        public bool IsMutable => _export.IsMutable;
+
+        public object Value
+        {
+            get
+            {
+                unsafe
+                {
+                    var v = stackalloc Interop.wasm_val_t[1];
+                    Interop.wasm_global_get(_global, v);
+                    return Interop.ToObject(v);
+                }
+            }
+            set
+            {
+                if (!IsMutable)
+                {
+                    throw new InvalidOperationException($"The value of global '{Name}' cannot be modified.");
+                }
+
+                var v = Interop.ToValue(value, Kind);
+
+                unsafe
+                {
+                    Interop.wasm_global_set(_global, &v);
+                }
+            }
+        }
+
+        private GlobalExport _export;
+        private IntPtr _global;
+    }
+}