fix potential unsafety bug for non-u8 types in macros

We don't have any non-u8 newtypes but better safe than sorry. See
https://github.com/apoelstra/rust-bitcoin/pull/45/

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 
 name = "secp256k1"
-version = "0.8.1"
+version = "0.8.2"
 authors = [ "Dawid Ciężarkiewicz <dpc@ucore.info>",
             "Andrew Poelstra <apoelstra@wpsoftware.net>" ]
 license = "CC0-1.0"

src/macros.rs

@@ -60,7 +60,7 @@ macro_rules! impl_array_newtype {
                     let mut ret: $thing = mem::uninitialized();
                     copy_nonoverlapping(self.as_ptr(),
                                         ret.as_mut_ptr(),
-                                        mem::size_of::<$thing>());
+                                        $len);
                     ret
                 }
             }