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/IHost.cs

@@ -14,12 +14,5 @@ namespace Wasmtime
         /// </summary>
         /// <remarks>A host can only bind to one module instance at a time.</remarks>
         Instance Instance { get; set; }
-
-        /// <summary>
-        /// Gets the import bindings of the host given a WebAssembly module.
-        /// </summary>
-        /// <param name="module">The WebAssembly module to get the import bindings for.</param>
-        /// <returns>Returns the list of import bindings for the host.</returns>
-        List<Binding> GetImportBindings(Module module) => Binding.GetImportBindings(this, module);
     }
 }