Modify internal string functions to support unicode strings.

Changed the way [] for a STRING and the builtin length function len()
work and made them respect unicode arrays (so we can peel off one
unicode character at a time rather than a useless char that is only 1/2
or 1/4 of a char).
Fix up write.scad to not use unicode explict hack anymore

With unicode awareness in strings now the write.scad just got a lot
cleaner. Made TestFont.scad a bit of a showpiece also.