Update string handling to handle SMP unicode blocks better.

[dpvc]

This PR updates the string-handling functions to take advantage of some ES6 string functions (fromCodePoint(), codePointAt(), and the fact that Array.from() splits a string into an array of unicode characters). It also enhances the TeX parser string handling to handle unicode characters better. E.g., you can use a (single) unicode character as a macro name (e.g., \α) or as an argument (e.g. \sqrt α) or in macro definition templates (e.g., \def\x #1α{[#1]} so that \x abcα will produce [abc]), and so on.

[zorkow]

Just some minor changes.