engine: add LoadAndRun method to reduce needed code to support most typical usage

[deadprogram]

This PR adds a LoadAndRun() method to the engine to reduce the boilerplate needed code to support the most typical usage.

[deadprogram]

With this change you can now do this:

    println("Loading and running WASM code...")
    ins, err := eng.LoadAndRun(bytes.NewReader(wasmCode))
    if err != nil {
        println(err.Error())
        return
    }

instead of always having to do this:

    println("Loading WASM module...")
    if err := eng.Interpreter.Load(bytes.NewReader(wasmModule)); err != nil {
        println(err.Error())
        return
    }

    println("Running module...")
    ins, err := eng.Interpreter.Run()
    if err != nil {
        println(err.Error())
        return
    }

[deadprogram]

Based on side-channel conversation with @orsinium we will also do some additional work in future PR to better scope Interpreter and Module for the individual Load() and Run() methods respectively.

[deadprogram]

Now merging.