Move header to payload front

I moved the header to the front of the payload buffer, like so:
```
+---------------+-------------------+
|   symbol id   |    symbol data    |
+---------------+-------------------+
```
I did so in the interest of making the API more simple.

This could potentially cause performance issues, but I haven't been able to get any definitive results by running  the benchmarks though.
I completely agree! :+1: