Small optimisation on top of this -- any boolean parameter can be pre-converted to '0' or '1'.

Further, for each symbol we can code whether than symbol can be flattened to a string if all its parameters are strings. This should be a very significant optimisation as a whole template instance may then become equivalent to a single string. If it is, we can set a flag in the object so when it gets attached to something else during optimise mode, it gets attached as string (i.e. evaluate gets called).