ap_expr: open string expressions to the <word>.

Introduces the syntax "%{:<word>:}", borrowed from the <var>'s one which is
"%{<var_name>[:<var_function_args>]}" and already allowed in string expressions
and which likewise can be embedded anywhere in a string expression (the same
reserved character ':' gets reused in an unambiguous manner).

This allows the two types of expressions (boolean and string) to now share
fully the same language set, namely: strings, lists, vars, regexes, backrefs,
functions with multiple or complex arguments, and especially combinations
thereof.

Most of them were reserved to boolean expressions only, while complex string
constructions can also benefit to, well, strings. The <word> construct allows
that (say the syntax "%{:<word>:}" looks like a temporary variable constructed
in a string).

Since string expressions may now have to deal with lists (arrays), they also
need a way to produce/extract strings from list and vice versa. This can be
done with the new "join" and "split" ...