My solution

Here's my solution (in src/main.rs). Hopefully that was pretty easy. You'll soon see that simple polling like this is not very practical.

#![no_main]
#![no_std]

use cortex_m_rt::entry;
use embedded_hal::digital::InputPin;
use microbit::{board::Board, display::blocking::Display, hal::Timer};
use panic_rtt_target as _;
use rtt_target::rtt_init_print;

// Define LED patterns
const LEFT_ARROW: [[u8; 5]; 5] = [
    [0, 0, 1, 0, 0],
    [0, 1, 0, 0, 0],
    [1, 1, 1, 1, 1],
    [0, 1, 0, 0, 0],
    [0, 0, 1, 0, 0],
];

const RIGHT_ARROW: [[u8; 5]; 5] = [
    [0, 0, 1, 0, 0],
    [0, 0, 0, 1, 0],
    [1, 1, 1, 1, 1],
    [0, 0, 0, 1, 0],
    [0, 0, 1, 0, 0],
];

const CENTER_LED: [[u8; 5]; 5] = [
    [0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0],
    [0, 0, 1, 0, 0],
    [0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0],
];

#[entry]
fn main() -> ! {
    rtt_init_print!();
    let board = Board::take().unwrap();
    let mut timer = Timer::new(board.TIMER0);

    let mut display = Display::new(board.display_pins);
    let mut button_a = board.buttons.button_a;
    let mut button_b = board.buttons.button_b;

    loop {
        if button_a.is_low().unwrap() {
            display.show(&mut timer, LEFT_ARROW, 10);
        } else if button_b.is_low().unwrap() {
            display.show(&mut timer, RIGHT_ARROW, 10);
        } else {
            display.show(&mut timer, CENTER_LED, 10);
        }
    }
}