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/ExternFunction.cs

@@ -0,0 +1,87 @@
+using System;
+using System.Collections.Generic;
+using Wasmtime.Exports;
+
+namespace Wasmtime.Externs
+{
+    /// <summary>
+    /// Represents an external (instantiated) WebAssembly function.
+    /// </summary>
+    public class ExternFunction
+    {
+        internal ExternFunction(FunctionExport export, IntPtr func)
+        {
+            _export = export;
+            _func = func;
+        }
+
+        /// <summary>
+        /// The name of the WebAssembly function.
+        /// </summary>
+        public string Name => _export.Name;
+
+        /// <summary>
+        /// The parameters of the WebAssembly function.
+        /// </summary>
+        public IReadOnlyList<ValueKind> Parameters => _export.Parameters;
+
+        /// <summary>
+        /// The results of the WebAssembly function.
+        /// </summary>
+        public IReadOnlyList<ValueKind> Results => _export.Results;
+
+        /// <summary>
+        /// Invokes the WebAssembly function.
+        /// </summary>
+        /// <param name="arguments">The array of arguments to pass to the function.</param>
+        /// <returns>
+        ///   Returns null if the function has no return value.
+        ///   Returns the value if the function returns a single value.
+        ///   Returns an array of values if the function returns more than one value.
+        /// </returns>
+        public object Invoke(params object[] arguments)
+        {
+            if (arguments.Length != Parameters.Count)
+            {
+                throw new WasmtimeException($"Argument mismatch when invoking function '{Name}': requires {Parameters.Count} but was given {arguments.Length}.");
+            }
+
+            unsafe
+            {
+                Interop.wasm_val_t* args = stackalloc Interop.wasm_val_t[Parameters.Count];
+                Interop.wasm_val_t* results = stackalloc Interop.wasm_val_t[Results.Count];
+
+                for (int i = 0; i < arguments.Length; ++i)
+                {
+                    args[i] = Interop.ToValue(arguments[i], Parameters[i]);
+                }
+
+                var trap = Interop.wasm_func_call(_func, args, results);
+                if (trap != IntPtr.Zero)
+                {
+                    throw TrapException.FromOwnedTrap(trap);
+                }
+
+                if (Results.Count == 0)
+                {
+                    return null;
+                }
+
+                if (Results.Count == 1)
+                {
+                    return Interop.ToObject(&results[0]);
+                }
+
+                var ret = new object[Results.Count];
+                for (int i = 0; i < Results.Count; ++i)
+                {
+                    ret[i] = Interop.ToObject(&results[i]);
+                }
+                return ret;
+            }
+        }
+
+        private FunctionExport _export;
+        private IntPtr _func;
+    }
+}