add invokeSync method

[nikpivkin]

This PR adds an invokeSync method that is similar to invoke, only executed synchronously.

The need for a synchronous method appeared for calling a WASM function from another WASM module. A small example:

async function main() {
    buffer = await fs.readFile(path.join(__dirname, 'wasm', 'main.wasm'));
    const host = await instantiate(buffer);

    const policyWasm = await fs.readFile("policy.wasm");
    const policy = await loadPolicy(policyWasm, {}, {
        "greet": () => (msg) => {
            console.log("Message: ", msg);
            const payload = encode(msg);
            console.log("Payload: ", payload);
    
            res = host.invokeSync('greet', payload)
            return result = decode(res);
        }
    })

    const result = policy.evaluate({});
    console.log(JSON.stringify(result, null, 2));
}