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/Module.cs

@@ -15,15 +15,13 @@ namespace Wasmtime
                 throw new ArgumentNullException(nameof(store));
             }
 
-            var bytesHandle = GCHandle.Alloc(bytes, GCHandleType.Pinned);
-
-            try
+            unsafe
             {
-                unsafe
+                fixed (byte *ptr = bytes)
                 {
                     Interop.wasm_byte_vec_t vec;
                     vec.size = (UIntPtr)bytes.Length;
-                    vec.data = (byte*)bytesHandle.AddrOfPinnedObject();
+                    vec.data = ptr;
 
                     Handle = Interop.wasm_module_new(store.Handle, ref vec);
                 }
@@ -33,10 +31,6 @@ namespace Wasmtime
                     throw new WasmtimeException($"WebAssembly module '{name}' is not valid.");
                 }
             }
-            finally
-            {
-                bytesHandle.Free();
-            }
 
             Store = store;
             Name = name;