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/Wasi.cs

@@ -0,0 +1,44 @@
+using System;
+using Wasmtime.Bindings;
+using Wasmtime.Imports;
+
+namespace Wasmtime
+{
+    public class Wasi
+    {
+        /// <summary>
+        /// Creates a default <see cref="Wasi"/> instance.
+        /// </summary>
+        public Wasi(Store store) :
+            this(
+                (store ?? throw new ArgumentNullException(nameof(store))).Handle,
+                Interop.wasi_config_new()
+            )
+        {
+        }
+
+        internal Wasi(Interop.StoreHandle store, Interop.WasiConfigHandle config)
+        {
+            IntPtr trap;
+            Handle = Interop.wasi_instance_new(store, config, out trap);
+            config.SetHandleAsInvalid();
+
+            if (trap != IntPtr.Zero)
+            {
+                throw TrapException.FromOwnedTrap(trap);
+            }
+        }
+
+        internal WasiBinding Bind(Import import)
+        {
+            var export = Interop.wasi_instance_bind_import(Handle, import.Handle);
+            if (export == IntPtr.Zero)
+            {
+                return null;
+            }
+            return new WasiBinding(export);
+        }
+
+        internal Interop.WasiInstanceHandle Handle { get; private set; }
+    }   
+}