fix rust-analyzer incorrect-ident-case diagnostic

Using the derive macro in a project with rust-analyzer gives an error
complaining that this generated code isn't in the SCREAMING_SNAKE_CASE
form. This change makes this diagnostic message go away.

derive/Cargo.toml

@@ -18,6 +18,7 @@ repository = "https://github.com/rust-fuzz/arbitrary"
 documentation = "https://docs.rs/arbitrary/"
 
 [dependencies]
+heck = "0.4"
 proc-macro2 = "1.0"
 quote = "1.0"
 syn = { version = "1.0", features = ['derive'] }

derive/src/lib.rs

@@ -1,5 +1,6 @@
 extern crate proc_macro;
 
+use heck::ToShoutySnakeCase;
 use proc_macro2::{Span, TokenStream};
 use quote::quote;
 use syn::*;
@@ -13,7 +14,7 @@ pub fn derive_arbitrary(tokens: proc_macro::TokenStream) -> proc_macro::TokenStr
         build_arbitrary_lifetime(input.generics.clone());
 
     let recursive_count = syn::Ident::new(
-        &format!("RECURSIVE_COUNT_{}", input.ident),
+        &format!("RECURSIVE_COUNT_{}", input.ident.to_string().to_shouty_snake_case()),
         Span::call_site(),
     );