Support WASI snapshot0 in the C API.

This commit adds support for snapshot0 in the WASI C API.

A name parameter was added to `wasi_instance_new` to accept which WASI module
is being instantiated.

Additionally, the C# API now supports constructing a WASI instance based on the
WASI module name.

Fixes #1221.

crates/misc/dotnet/src/Interop.cs

@@ -936,6 +936,7 @@ namespace Wasmtime
         [DllImport(LibraryName)]
         public static extern WasiInstanceHandle wasi_instance_new(
             StoreHandle store,
+            [MarshalAs(UnmanagedType.LPUTF8Str)] string name,
             WasiConfigHandle config,
             out IntPtr trap
         );