Support char -> string similarly to T -> seq[T]

[schneiderfelipe]

That shows up when things get nested, e.g., when we start getting things like seq[seq[char]], which should be seq[string], since seq[char] is close enough to string, but seq[seq[char]] is not comparable to seq[string].

EDIT: it might not scale well to overload each procedure, so it would be nice to have templates to define both functions every time. The idea is to have every procedure that returns seq[T] to return string when T = char, using simple placeholders.