Add example for saving (#470)

Adds a quick example for saving a some numbers in sram.

- [x] no changelog update needed

agb/examples/save.rs

@@ -0,0 +1,29 @@
+#![no_std]
+#![no_main]
+
+use agb::save::Error;
+
+extern crate alloc;
+use alloc::vec::Vec;
+
+#[agb::entry]
+fn main(mut gba: agb::Gba) -> ! {
+    test_save(gba).unwrap();
+    panic!("example finished");
+}
+
+fn test_save(mut gba: agb::Gba) -> Result<(), Error> {
+    gba.save.init_sram();
+    let mut access = gba.save.access()?;
+
+    let mut is_save = 0;
+    access.read(0, core::slice::from_mut(&mut is_save))?;
+
+    if is_save != 0 {
+        access
+            .prepare_write(0..128)?
+            .write(0, &(0..128).collect::<Vec<_>>())?;
+    }
+
+    Ok(())
+}