context: introduce unsafe PreallocatedContext trait

Fixes unsoundness in `preallocated_gen_new` which previously did not
properly constrain the lifetime of the buffer used to back the context
object. We introduce an unsafe marker trait, and impl it for our
existing preallocated-context markers.

Annoyingly the trait has to be public even though it should never be
used directly, and is only used alongside the sealed `Context` trait,
so it is de-facto sealed itself.

Fixes #543

src/context.rs

@@ -318,7 +318,15 @@ unsafe impl<'buf> Context for AllPreallocated<'buf> {
     }
 }
 
-impl<'buf, C: Context + 'buf> Secp256k1<C> {
+/// Trait marking that a particular context object internally points to
+/// memory that must outlive `'a`
+pub unsafe trait PreallocatedContext<'a> {}
+
+unsafe impl<'buf> PreallocatedContext<'buf> for AllPreallocated<'buf> {}
+unsafe impl<'buf> PreallocatedContext<'buf> for SignOnlyPreallocated<'buf> {}
+unsafe impl<'buf> PreallocatedContext<'buf> for VerifyOnlyPreallocated<'buf> {}
+
+impl<'buf, C: Context + PreallocatedContext<'buf>> Secp256k1<C> {
     /// Lets you create a context with a preallocated buffer in a generic manner (sign/verify/all).
     pub fn preallocated_gen_new(buf: &'buf mut [AlignedType]) -> Result<Secp256k1<C>, Error> {
         #[cfg(target_arch = "wasm32")]