Generate a random String
Returns a generator spec producing Strings whose characters are drawn
from chars (a String, List, or Vector of Strings) with
lengths between min_len and max_len
inclusive.
chars (String|List[String]):
Candidate characters.
min_len (Int): Minimum length
(inclusive).
max_len (Int): Maximum length
(inclusive).
A generator spec.
assert(prop_for_all(prop_gen_string_from("ab", 1, 3), \(s) length(s) <= 3))