Derive Debug for a few types

Found by clippy.

Author: tcharding

src/context/internal_nostd.rs

@@ -17,6 +17,7 @@ mod self_contained_context {
     const MAX_PREALLOC_SIZE: usize = 16; // measured at 208 bytes on Andrew's 64-bit system
 
     /// A secp256k1 context object which can be allocated on the stack or in static storage.
+    #[derive(Debug)]
     pub struct SelfContainedContext(
         [MaybeUninit<AlignedType>; MAX_PREALLOC_SIZE],
         Option<NonNull<ffi::Context>>,

src/context/spinlock.rs

@@ -23,6 +23,7 @@ const MAX_SPINLOCK_ATTEMPTS: usize = 128;
 // new stack-local context object if we are unable to obtain a lock on the
 // global one. This is slow and loses the defense-in-depth "rerandomization"
 // anti-sidechannel measure, but it is better than deadlocking..
+#[derive(Debug)]
 pub struct SpinLock<T> {
     flag: AtomicBool,
     // Invariant: if this is non-None, then the store is valid and can be
@@ -83,6 +84,7 @@ impl<T> SpinLock<T> {
 }
 
 /// Drops the lock when it goes out of scope.
+#[derive(Debug)]
 pub struct SpinLockGuard<'a, T> {
     lock: &'a SpinLock<T>,
 }