It finally works now
Definitely some clean up to be done

Currently writes IHDR and IEND chunks correctly
CRC implementation is borrowed from the specification
Writer class now also has a buffer for the CRC calculation

Also moved both the reader and writer class to the same files, named
files.{cpp,h}

Also improved error handling slightly

This is so I can test PNG writing by reading a BMP and writing it to a
PNG, effectively the reverse of the PNG reading testing

Currently not incredibly well implemented, but it can handle fixed
codes

All functions and classes are now behind the cpp namespace `TehImage`

Also added the bitmap image subclass
note: this is pretty hacky in how it works

Bringing a lot of stuff over from my maze-reader project in an attempt
to split the image reading code off into a library.