Setup clippy (#349)

Setup clippy

src/dc_array.rs

@@ -403,5 +403,4 @@ mod tests {
         }
         unsafe { dc_array_get_id(arr, 1000) };
     }
-
 }

src/lib.rs

@@ -1,4 +1,9 @@
-#![feature(c_variadic, ptr_wrapping_offset_from, ptr_cast)]
+#![deny(clippy::correctness)]
+// TODO: make all of these errors, such that clippy actually passes.
+#![warn(clippy::all, clippy::perf, clippy::not_unsafe_ptr_arg_deref)]
+// This is nice, but for now just annoying.
+#![allow(clippy::unreadable_literal)]
+#![feature(ptr_wrapping_offset_from)]
 
 #[macro_use]
 extern crate failure_derive;