Port multisig example to low level data structures (use-ink#1078)

* Fix `Default` impl of `Mapping`

* Replace HashMap by Mapping

* Use initialize_contract

* Initialize everything within `initialize_contract`

* Make owners lazy

* Remove commented out code

There's no way to get the length of a `Mapping` so it doesn't make sense
to check this in tests anymore

* Fix - + bug

* No need to make a primitive lazy

* Clippy

Co-authored-by: Hernando Castano <hernando@hcastano.com>

crates/storage/src/lazy/mapping.rs

@@ -38,12 +38,21 @@ use ink_primitives::Key;
 
 /// A mapping of key-value pairs directly into contract storage.
 #[cfg_attr(feature = "std", derive(scale_info::TypeInfo))]
-#[derive(Default)]
 pub struct Mapping<K, V> {
     offset_key: Key,
     _marker: PhantomData<fn() -> (K, V)>,
 }
 
+/// We implement this manually because the derived implementation adds trait bounds.
+impl<K, V> Default for Mapping<K, V> {
+    fn default() -> Self {
+        Self {
+            offset_key: Default::default(),
+            _marker: Default::default(),
+        }
+    }
+}
+
 impl<K, V> core::fmt::Debug for Mapping<K, V> {
     fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
         f.debug_struct("Mapping")