clean up the design of Char

[bobzhang]

Char is a unicode code point in MoonBit, it is encoding agnostics, it is not tied to utf16, so we should not provide APIs like this:

fn Char::is_surrogate(Char) -> Bool

We should provide String.as_iter which returns Iter[Char]

“A😊𠮷B"
   .as_iter()
   .reduce(init=Vec::[], fn {acc, x -> acc.push(x)})
// expected output
[65, 128522, 134071, 66]

cc @yj-qin