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/context.rs

@@ -533,6 +533,7 @@ pub fn dc_get_fresh_msgs(context: &Context) -> *mut dc_array_t {
         .unwrap()
 }
 
+#[allow(non_snake_case)]
 pub fn dc_search_msgs(
     context: &Context,
     chat_id: uint32_t,