chore!: define key type in maps (#3841)

Closes AztecProtocol/aztec-packages#2787

Unfortunately key type with a `ToField` trait binding is only defined in
the `.at` method, due to this blocker:

noir-lang/noir#3954

authwit/src/account.nr

@@ -8,7 +8,7 @@ use crate::auth::IS_VALID_SELECTOR;
 struct AccountActions {
   context: Context,
   is_valid_impl: fn(&mut PrivateContext, Field) -> bool,
-  approved_action: Map<PublicState<bool, BOOL_SERIALIZED_LEN>>,
+  approved_action: Map<Field, PublicState<bool, BOOL_SERIALIZED_LEN>>,
 }
 
 impl AccountActions {

aztec/src/state_vars/map.nr

@@ -1,22 +1,25 @@
 use crate::context::{PrivateContext, PublicContext, Context};
 use dep::std::option::Option;
-use dep::protocol_types::hash::pedersen_hash;
+use dep::protocol_types::{
+    hash::pedersen_hash,
+    traits::{ToField}
+};
 
 // docs:start:map
-struct Map<V> {
+struct Map<K, V> {
     context: Context,
     storage_slot: Field,
     state_var_constructor: fn(Context, Field) -> V,
 }
 // docs:end:map
 
-impl<V> Map<V> {
+impl<K, V> Map<K, V> {
     // docs:start:new
     pub fn new(
         context: Context,
         storage_slot: Field,
         state_var_constructor: fn(Context, Field) -> V,
-    ) -> Map<V> {
+    ) -> Self {
         assert(storage_slot != 0, "Storage slot 0 not allowed. Storage slots must start from 1.");
         Map {
             context,
@@ -27,9 +30,9 @@ impl<V> Map<V> {
     // docs:end:new
 
     // docs:start:at
-    pub fn at(self, key: Field) -> V {
+    pub fn at(self, key: K) -> V where K: ToField {
         // TODO(#1204): use a generator index for the storage slot
-        let derived_storage_slot = pedersen_hash([self.storage_slot, key],0);
+        let derived_storage_slot = pedersen_hash([self.storage_slot, key.to_field()],0);
 
         let state_var_constructor = self.state_var_constructor;
         state_var_constructor(self.context, derived_storage_slot)