Use longer lifetime in clap_lex ParsedArg

[sffc]

It seems that we can return 's lifetimes here. It allows for easier zero-copy args parsing, i.e., writing an args parser with a signature such as

fn parse_args<'s>(args: &'s RawArgs) -> Cli<'s> { ... }

Currently args.next() returns ParsedArg<'s>, but ParsedArg::to_value_os(&'x self) returns &'x OsStr when it can return &'s OsStr.