context: whitelist new compiler warning

The compiler doesn't like shared references to static mut variables,
because it is worried that we'll share a reference while a mutation
happens which is UB.

We are only sharing references after using Once to do a one-time
initialization, so this is fine, but in recent versions of the compiler
there is an encapsulated form of this pattern called `OnceLock` and a
variant called `LazyLock`.

Neither of these are available on our MSRV so just whitelist the lint.

Author: apoelstra

src/context.rs

@@ -42,6 +42,8 @@ pub mod global {
         type Target = Secp256k1<All>;
 
         #[allow(unused_mut)] // Unused when `rand` + `std` is not enabled.
+        #[allow(static_mut_refs)] // The "proper" way to do this is with OnceLock (MSRV 1.70) or LazyLock (MSRV 1.80)
+                                  // See https://doc.rust-lang.org/nightly/edition-guide/rust-2024/static-mut-references.html
         fn deref(&self) -> &Self::Target {
             static ONCE: Once = Once::new();
             static mut CONTEXT: Option<Secp256k1<All>> = None;