Move to a generic GlobalLazy<T> (#2851)

* Expose a generic lazy type

* Switch to generic lazy globals

* simplify global lazy context a bit

* rename LazyGlobal to Global

* use Memo::global in more examples

* Fix soundness issue with deref_impl. It relies on the size of self, so it cannot be safe

* add a comment about safety

* Make clippy happy

* fix formatting

* Restore changes to signal impl

* Add helper methods for global signal and global memo to make getting the inner value easier

packages/signals/src/read.rs

@@ -221,8 +221,11 @@ pub trait Readable {
         <Self::Storage as AnyStorage>::map(self.read(), |v| v.index(index))
     }
 
+    /// SAFETY: You must call this function directly with `self` as the argument.
+    /// This function relies on the size of the object you return from the deref
+    /// being the same as the object you pass in
     #[doc(hidden)]
-    fn deref_impl<'a>(&self) -> &'a dyn Fn() -> Self::Target
+    unsafe fn deref_impl<'a>(&self) -> &'a dyn Fn() -> Self::Target
     where
         Self: Sized + 'a,
         Self::Target: Clone,