Calls a function with self.
ie. a.loose_map(b) = b(a).
Declares a trait/method similar to std::iter::Iterator::map,
where any type that is the sole argument for a function can be
called into such function.
ps. I called this "being mapped into such function" but I don't know if this is correct.
This is an example of usage with the loosen attr derive macro.
use loosen::loose;
use ow::LooseMap;
#
# struct A;
# struct B;
#[loose]
fn fa(a: A, b: B) -> bool { true }
// normal call
assert!(fa(A, B));
// loose call, available from the `loosen` crate
assert!(fa_loose((A, B)));
// loose_map with loose call
assert!((A, B).loose_map(fa_loose));
// ie. from the argument (or loosened arguments),
// you may call some function (or loosened function)