Improve XIO ROM sample

[networkfusion]

Description

Improves buildability, function names, ROM text and source comments (and formatting) for the XIO sample. Given that it is based on #17 , I would expect that to be merged before releasing this one (so it can be squashed correctly)

Related Issue

There is none.

Motivation and Context

The sample was previously un-buildable for most users, and the source was hard to read. I have improved the source as far as possible without renaming files (that will come in a later PR)

Further improvements for later would include making an ED64 lib and renaming source files, but it is best this is merged first.

How Has This Been Tested?

Loading the built ROM onto real hardware containing a V3 cart with V3.05 OS

Screenshots (if appropriate):