Fix atoms module name generated by the NifStruct derive macro (#585)

This is related to #573, but
now the macro is given a warning even with the annotation to ignore.

So this is a fix that turns the atoms module name generated by the macro
in a snake case name.

rustler_codegen/src/context.rs

@@ -86,7 +86,10 @@ impl<'a> Context<'a> {
     }
 
     pub fn atoms_module_name(&self, span: Span) -> Ident {
-        Ident::new(&format!("RUSTLER_ATOMS_{}", self.ident), span)
+        Ident::new(
+            &format!("rustler_atoms_{}", self.ident).to_snake_case(),
+            span,
+        )
     }
 
     pub fn encode(&self) -> bool {

rustler_codegen/src/ex_struct.rs

@@ -53,7 +53,6 @@ pub fn transcoder_decorator(ast: &syn::DeriveInput, add_exception: bool) -> Toke
     };
 
     let gen = quote! {
-        #[allow(non_snake_case)]
         mod #atoms_module_name {
             #atom_defs
         }