read_char(s = current_reader())
Return the next character from textual input stream s.
If the stream has no more characters left, return the end-of-file object.
let f = file_reader("README.md")
letfn loop (c = read_char(f), cs = [])
if (is_eof_object(c)) reverse(cs)
else loop(read_char(f), c:cs)
// [a list of characters...]
close_reader(f)