Function tokenize 

pub fn tokenize(args: &[String]) -> Vec<Token>

Tokenize a slice of CLI arguments into a vector of typed tokens.

Rules:

1. -- alone emits EndOfOptions; everything after becomes Positional.
2. --name or --name=val emits LongFlag.
3. -x emits ShortFlag; -x=val emits ShortFlag with value; -abc (len > 2, no =) emits ShortCluster.
4. - alone or -42 (dash + digit) emits Positional.
5. Everything else emits Positional.