Fix #2612: adjust readable trait to allow try_peek (#2714)

* Fix #2612: adjust readable trait to allow try_peek

Our implementation for Readable was inconsistent.
We had a try_unchecked variant for read but not for peek.
This resolves that by making a breaking change to the
Readable interface.

* fix doctests too

packages/signals/src/copy_value.rs

@@ -1,5 +1,5 @@
-use generational_box::GenerationalBoxId;
 use generational_box::UnsyncStorage;
+use generational_box::{BorrowResult, GenerationalBoxId};
 use std::ops::Deref;
 
 use dioxus_core::prelude::*;
@@ -129,8 +129,8 @@ impl<T: 'static, S: Storage<T>> Readable for CopyValue<T, S> {
     }
 
     #[track_caller]
-    fn peek_unchecked(&self) -> ReadableRef<'static, Self> {
-        self.value.read()
+    fn try_peek_unchecked(&self) -> BorrowResult<ReadableRef<'static, Self>> {
+        self.value.try_read()
     }
 }