Procedural macro for fixed-exponent variable-base modular exponentiation

Uses the addchain crate to obtain an addition chain for the exponent,
and then generates the corresponding constant-time square-and-multiply
algorithm.

ff/ff_derive/src/lib.rs

@@ -10,6 +10,8 @@ use quote::quote;
 use quote::TokenStreamExt;
 use std::str::FromStr;
 
+mod pow_fixed;
+
 #[proc_macro_derive(PrimeField, attributes(PrimeFieldModulus, PrimeFieldGenerator))]
 pub fn prime_field(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
     // Parse the type definition