Add a wasmtime-specific wasmtime_wat2wasm C API (#1206)

* Add a wasmtime-specific `wasmtime_wat2wasm` C API

This commit implements a wasmtime-specific C API for converting the text
format to the binary format. An upstream spec issue exists for adding
this to the C API, but in the meantime we can experiment with our own
version of this API and use it in the C# extension, for example!

Closes #1000

* Reorder arguments

* Use wasm_byte_vec_t for input `*.wat`

* Mark wat input as const

* Return an error message and use `fixed`

* Actually include the error message

* Use `fixed` in `Module.cs` as well

crates/misc/dotnet/tests/Fixtures/ModuleFixture.cs

@@ -13,7 +13,9 @@ namespace Wasmtime.Tests
                 .WithReferenceTypes(true)
                 .Build();
             Store = Engine.CreateStore();
-            Module = Store.CreateModule(Path.Combine("Modules", ModuleFileName));
+            var wat = Path.Combine("Modules", ModuleFileName);
+            var wasm = Engine.WatToWasm(File.ReadAllText(wat));
+            Module = Store.CreateModule(wat, wasm);
         }
 
         public void Dispose()