Update issue_1 error message

Author: Pat-Lafon

hashconsing/tests/try_build/issue_1.stderr

@@ -22,14 +22,19 @@ error[E0277]: `Cell<RefOrInt<'_>>` cannot be shared between threads safely
    | |_________^ `Cell<RefOrInt<'_>>` cannot be shared between threads safely
    |
    = help: within `&HashableCell<RefOrInt<'_>>`, the trait `Sync` is not implemented for `Cell<RefOrInt<'_>>`
+   = note: if you want to do aliasing and mutation between multiple threads, use `std::sync::RwLock`
 note: required because it appears within the type `HashableCell<RefOrInt<'_>>`
   --> tests/try_build/issue_1.rs:18:8
    |
 18 | struct HashableCell<T: Eq + PartialEq + Copy> {
    |        ^^^^^^^^^^^^
    = note: required because it appears within the type `&HashableCell<RefOrInt<'_>>`
    = note: required for `Arc<&HashableCell<RefOrInt<'_>>>` to implement `Send`
-   = note: required because it appears within the type `HConsed<&HashableCell<RefOrInt<'_>>>`
+note: required because it appears within the type `HConsed<&HashableCell<RefOrInt<'_>>>`
+  --> src/lib.rs
+   |
+   | pub struct HConsed<T> {
+   |            ^^^^^^^
 note: required because it's used within this closure
   --> tests/try_build/issue_1.rs:37:17
    |
@@ -38,5 +43,8 @@ note: required because it's used within this closure
 note: required by a bound in `crossbeam_utils::thread::Scope::<'env>::spawn`
   --> $CARGO/crossbeam-utils-0.8.15/src/thread.rs
    |
+   |     pub fn spawn<'scope, F, T>(&'scope self, f: F) -> ScopedJoinHandle<'scope, T>
+   |            ----- required by a bound in this associated function
+...
    |         F: Send + 'env,
    |            ^^^^ required by this bound in `Scope::<'env>::spawn`