type signature in statusline

[Aras14HD]

Add a statusline option for showing the type signature of the variable or function under the cursor. (I don't wanna press Space k every time)

Variable example: alloc::vec::Vec<Option<f64>>

Function example: name<T: Iterator>(arg: &T) -> T::Item where T::Item: PartialOrd

The above is probably the most verbose case possible. These should be easily taken from the doc (It's displayed mostly as the first line).

I have had this in other editors (doom emacs), and I think it's very helpful to see at at a glance.