Add way to print sequences of `Key`s to strings about sapling

[kneasle]

It's here . The rest of the related code should be reachable easily by jumping to definition a load of times.

[stokhos]

I'd like to fix this.
where do you want the string to be printed?

[kneasle]

The main reason we need this is so that #80 doesn't mess up the readout in the bottom right corner, as seen here:

What would be ideal is to have a function which takes a slice of Keys and converts them into a string by calling core::key_display::KeyDisplay::compact_string on each of them in turn. We can then use it for the bottom-right-corner readout, and the key stroke list in the keystroke log.

I think it would be tidy to add this function to KeyDisplay too.

[stokhos]

Can you show me the code bock for the bottom-right-corner readout? I couldn't find it.

[stokhos]

I think it would be tidy to add this function to KeyDisplay too.

Can you be more specific over this

pub fn keystrokes_to_string(keystroke_buffer: &Vec<Key>) -> String {
    keystroke_buffer
        .iter()
        .map(|x| x.compact_string())
        .collect()
}

This is my implementation, and it won't work with KeyDisplay