Added the times operator for regular expressions, such that one can express
/[a-z]{10}/ in this way: `Regex = range('a', 'z') * 10'.

extras/lex/lex.m:
    Removed unused and unsafe str_foldr function,
    added (T * int) = regexp function.

extras/lex/samples/lex_demo.m:
    Removed whitespace in comments,
    added an input prompt,
    added a lexeme for '//' C++ comments using the new '*' operator.
Merge branch 'review-add_times-operator-for-liblex' of https://github.com/sebgod/mercury into review-add_times-operator-for-liblex

Conflicts:
	extras/lex/samples/lex_demo.m