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/tests/Fixtures/ModuleFixture.cs

@@ -15,19 +15,19 @@ namespace Wasmtime.Tests
 
         public void Dispose()
         {
-            if (Module != null)
+            if (!(Module is null))
             {
                 Module.Dispose();
                 Module = null;
             }
 
-            if (Store != null)
+            if (!(Store is null))
             {
                 Store.Dispose();
                 Store = null;
             }
 
-            if (Engine != null)
+            if (!(Engine is null))
             {
                 Engine.Dispose();
                 Engine = null;