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/examples/hello/Program.cs

@@ -18,13 +18,12 @@ namespace HelloExample
     {
         static void Main(string[] args)
         {
-            using (var engine = new Engine())
-            using (var store = engine.CreateStore())
-            using (var module = store.CreateModule("hello.wasm"))
-            using (dynamic instance = module.Instantiate(new Host()))
-            {
-                instance.run();
-            }
+            using var engine = new Engine();
+            using var store = engine.CreateStore();
+            using var module = store.CreateModule("hello.wasm");
+            using dynamic instance = module.Instantiate(new Host());
+
+            instance.run();
         }
     }
 }