Build a caching stringformatter

[JonnyOThan]

A lot of time is spent assembling strings from variables etc. It might be better to pre-parse the insertion points etc and have a more efficient way to build the strings rather than recalculating it from scratch every time

[JonnyOThan]

In addition it might be smart to cache the last output string, and only update it if the variables changed.

NOTE: this has now been done.