feat(menu): add a simple boot menu similar to systemd-boot (#100)

The boot menu is now a BootMenu trait with two styles. The basic menu is
the existing one that prints entries and selects them by number. The new
simple menu is full-screen: entries are centered and highlighted, the
arrow keys, home, end, page up and page down move the selection, enter
boots it, and any key stops the countdown.

The style is picked with --menu-style or menu-style in [options], and
defaults to simple. The timer code used to wait for a key is shared
between both menus in menu.rs.

README.md

@@ -120,6 +120,8 @@ $ sprout.efi --config=\path\to\config.toml
 $ sprout.efi --boot="Boot Xen"
 # Autoconfigure Sprout, without loading a configuration file.
 $ sprout.efi --autoconfigure
+# Use the basic boot menu instead of the simple one.
+$ sprout.efi --menu-style=basic
 ```
 
 ### Boot Linux from ESP