Move string handling on top of str object

[katajakasa]

Currently we use raw char pointers. Instead we could/should use the str object.

This would allow nicer handling of the strings in current/future tools and hopefully cleaner code. This would also allow us to abstract away the string handling to some safer library later and/or move to Unicode with utf-8 encoding -- better translations further down the line ?

Note that for the parser code, this requires #400 to be done first. Elsewhere this could already be (partially) done.