Function mail_internals::grammar::is_ftext

pub fn is_ftext(ch: char) -> bool

ftext as defined by RFC 5322

which is: printable US-ASCII characters not includign : => 0x21-0x39 / 0x3B-0x7E => '!'...'9' / ';'...'~' => <0x7F && != 0x3A