miri-script: use a proper CLI arg parsing tool

[RalfJung]

We are currently using ad-hoc hand-written parsers for this, and it's kind of annoying. However, we tried to use clap, and it currently doesn't seem possible: what we'd like to do is that once there is a positional argument, that argument and everything that follows gets collected in Vec -- our flags, like --dep, need to come before all of them.

This is non-standard, as it makes ./miri run foo.rs --dep different from ./miri run --dep foo.rs. But it's kind of necessary as ./miri run (and a bunch of other commands) are wrappers around other, complicated tools for which we want to just forward all flags (including --) without parsing them ourselves.

Using clap here is blocked on clap-rs/clap#5055. But maybe there's another crate we can use?

[RalfJung]

With #3621, our parser's behavior changed: ./miri run foo.rs --dep and ./miri run --dep foo.rs are now equivalent. However, we'd still like to preserve the -- and forward it to miri (e.g. in ./miri run foo.rs -- arg1 arg2), so we still can't use clap.