Validate wallet passwords as UTF-8

[michaelsproul]

Description

Presently we accept arbitrary bytes from a file as a password, which doesn't seem like the most intuitive UX, particularly when we want to treat it like text and strip out newline characters as per #1175. Making the PlainText type contain an actual String would make the newline stripping a lot cleaner too.

[michaelsproul]

For reference, newline stripping becomes as straightforward as:

fn main() {
    println!("{:?}", "mysupersecurepassword\r\n".trim_end_matches(|c| c == '\r' || c == '\n'));
    println!("{:?}", "mysupersecurepassword\r\n\r\n\r\r".trim_end_matches(|c| c == '\r' || c == '\n'));
}

Playground Link

Which is simpler than working with bytes as in #1199

[paulhauner]

Makes sense to me!

[roynalnaruto]

Description

Presently we accept arbitrary bytes from a file as a password, which doesn't seem like the most intuitive UX, particularly when we want to treat it like text and strip out newline characters as per #1175. Making the PlainText type contain an actual String would make the newline stripping a lot cleaner too.

Oh, I saw this issue only after you had merged the previous PR. Anyway, I have made the update to support UTF-8 here