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/src/Engine.cs

@@ -1,4 +1,6 @@
 using System;
+using System.Text;
+using System.Runtime.InteropServices;
 
 namespace Wasmtime
 {
@@ -40,6 +42,34 @@ namespace Wasmtime
             return new Store(this);
         }
 
+        /// <summary>
+        /// Converts the WebAssembly text format to the binary format
+        /// </summary>
+        /// <returns>Returns the binary-encoded wasm module.</returns>
+        public byte[] WatToWasm(string wat)
+        {
+            var watBytes = Encoding.UTF8.GetBytes(wat);
+            unsafe
+            {
+                fixed (byte *ptr = watBytes)
+                {
+                    Interop.wasm_byte_vec_t watByteVec;
+                    watByteVec.size = (UIntPtr)watBytes.Length;
+                    watByteVec.data = ptr;
+                    if (!Interop.wasmtime_wat2wasm(Handle, ref watByteVec, out var bytes, out var error)) {
+                        var errorSpan = new ReadOnlySpan<byte>(error.data, checked((int)error.size));
+                        var message = Encoding.UTF8.GetString(errorSpan);
+                        Interop.wasm_byte_vec_delete(ref error);
+                        throw new WasmtimeException("failed to parse input wat: " + message);
+                    }
+                    var byteSpan = new ReadOnlySpan<byte>(bytes.data, checked((int)bytes.size));
+                    var ret = byteSpan.ToArray();
+                    Interop.wasm_byte_vec_delete(ref bytes);
+                    return ret;
+                }
+            }
+        }
+
         /// <inheritdoc/>
         public void Dispose()
         {