Implement WASI C API.

This commit implements an initial WASI C API that can be used to instantiate
and configure a WASI instance from C.

This also implements a `WasiBuilder` for the C# API enabling .NET hosts to bind
to Wasmtime's WASI implementation.

crates/misc/dotnet/src/Bindings/WasiBinding.cs

@@ -0,0 +1,23 @@
+using System;
+using System.Runtime.InteropServices;
+
+namespace Wasmtime.Bindings
+{
+    /// <summary>
+    /// Represents a binding to a WASI export.
+    /// </summary>
+    internal class WasiBinding : Binding
+    {
+        public WasiBinding(IntPtr handle)
+        {
+            _handle = handle;
+        }
+
+        public override SafeHandle Bind(Store store, IHost host)
+        {
+            return new Interop.WasiExportHandle(_handle);
+        }
+
+        private IntPtr _handle;
+    }
+}