Override `non_shake_case' warning on per-function basis

This change removes global override of `non_shake_case' warning and replaces it
with per-function overrrides. This way, compiler will complain about style
guide violation in new code.

It should be noted, that `rustc' is not smart enough to emit warning when
override is no longer needed, it must be checked manually.

src/dc_e2ee.rs

@@ -54,6 +54,7 @@ impl Default for dc_e2ee_helper_t {
     }
 }
 
+#[allow(non_snake_case)]
 pub unsafe fn dc_e2ee_encrypt(
     context: &Context,
     recipients_addr: *const clist,