`Term.with_used_args` is very buggy

[dbuenzli]

Term.with_used_args which discussed in #101 and implemented in #103 has many bugs.

1. It reorders positional and optional arguments; that could be fine.
2. It separates values from short option names. This is a bug.
3. It seperates values from long option names. That could be fine.
4. It does not report the -- token. This is a bug.

Shown here on the current test exectuble test_with_used_args

> b0 -- test_with_used_args -- hoho --bbb=bla
hoho --bbb=bla
> b0 -- test_with_used_args hoho -bbla
-b bla hoho
> b0 -- test_with_used_args hoho --bbb=bla
--bbb bla hoho
> b0 -- test_with_used_args -- hoho --bbb=bla
hoho --bbb=bla