Make event_subscriptions a HashedMapView. (#4390)

## Motivation

This is part of the execution state view, so we should cache its hash.

## Proposal

Make it a `HashedMapView`.

## Test Plan

CI; I believe this should not change semantics, only slightly improve
performance.

## Release Plan

- Nothing to do / These changes follow the usual release cycle.

## Links

- [reviewer
checklist](https://github.com/linera-io/linera-protocol/blob/main/CONTRIBUTING.md#reviewer-checklist)

Author: afck

linera-execution/src/system.rs

@@ -24,7 +24,7 @@ use linera_base::{
 };
 use linera_views::{
     context::Context,
-    map_view::{HashedMapView, MapView},
+    map_view::HashedMapView,
     register_view::HashedRegisterView,
     set_view::HashedSetView,
     views::{ClonableView, HashableView, ReplaceContext, View},
@@ -90,7 +90,7 @@ pub struct SystemExecutionStateView<C> {
     /// Blobs that have been used or published on this chain.
     pub used_blobs: HashedSetView<C, BlobId>,
     /// The event stream subscriptions of applications on this chain.
-    pub event_subscriptions: MapView<C, (ChainId, StreamId), EventSubscriptions>,
+    pub event_subscriptions: HashedMapView<C, (ChainId, StreamId), EventSubscriptions>,
 }
 
 impl<C: Context, C2: Context> ReplaceContext<C2> for SystemExecutionStateView<C> {