Printing of closure types and MIR closure constructors

[oli-obk]

Quoting @eddyb in #66850 (comment)

I'm not that happy with [closure@PATH::{{closure}}#n ...], but it's consistent with what we have so far, if we want to go further I'd prefer something like this:

foo<T>::{closure#0} { q:&i32, t:&T }

(yes, {{closure}}#n -> {closure#n} is a change we should do eventually, IMO)
and without -Z span_free_formats we could perhaps go for:

closure(src/main.rs:10:13) { q:&i32, t:&T }

(and we need to remember to fix generators as well)

Oh and we could also change fn definition types from:

fn(bool) -> bool {std::convert::identity::<bool>}

to:

fn std::convert::identity::<bool>(bool) -> bool

[arora-aman]

If I understand correctly First part implemented here: #76176