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_simplify.rs

@@ -61,6 +61,7 @@ impl dc_simplify_t {
     /**
      * Simplify Plain Text
      */
+    #[allow(non_snake_case)]
     unsafe fn simplify_plain_text(
         &mut self,
         buf_terminated: *const libc::c_char,