Merge rust-bitcoin/rust-secp256k1#345: Add a static immutable zero aligned type

5e6d0f1363 Switch to associated constant (Jonathan Underwood)
9cf552e240 Add a static immutable zero aligned type (junderw)

Pull request description:

  The `zeroed` fn can not be used in static assignments.

  In environments where it is no_std and no allocator are present, the only way to get a slice of AlignedTypes is dynamically, so `preallocated_gen_new` can't be used.

  By offering this as a static, it can be used in static assignments as such:

  ```rust
  #[cfg(target_pointer_width = "32")]
  static mut CONTEXT_BUFFER: [AlignedType; 69645] = [ZERO_ALIGNED; 69645];
  #[cfg(target_pointer_width = "64")]
  static mut CONTEXT_BUFFER: [AlignedType; 69646] = [ZERO_ALIGNED; 69646];
  static mut SECP256K1: Option<Secp256k1<AllPreallocated>> = None;

  pub fn get_context(seed: Option<&[u8; 32]>) -> &'static Secp256k1<AllPreallocated<'static>> {
      unsafe {
          if SECP256K1.is_none() {
              SECP256K1 = Some(
                  Secp256k1::preallocated_gen_new(&mut CONTEXT_BUFFER)
                      .expect("CONTEXT_BUFFER size is wrong"),
              );
          }
          if let Some(seed) = seed {
              SECP256K1.as_mut().unwrap().seeded_randomize(seed);
          }
          SECP256K1.as_ref().unwrap()
      }
  }
  ```

ACKs for top commit:
  apoelstra:
    ACK 5e6d0f1363

Tree-SHA512: fc800f8c5c637fc7f81312da17f0a96d17cd087a2e6876f4dedbefffbe92b3625deb93636265f334f9fbd7ac38baa529d4ec72857dae662e26d753f32f91d394

secp256k1-sys/src/types.rs

@@ -40,6 +40,9 @@ impl AlignedType {
     pub fn zeroed() -> Self {
         AlignedType([0u8; 16])
     }
+
+    /// A static zeroed out AlignedType for use in static assignments of [AlignedType; _]
+    const ZERO: AlignedType = AlignedType([0u8; 16]);
 }
 
 #[cfg(all(feature = "std", not(rust_secp_no_symbol_renaming)))]