constant_unsigned_array31 macro needs to be macro exported

[M-ArafatZaman]

The fp31! macro is used to define a new field where you can specify the number of limbs (among other things).

constant_unsigned_array31! { 9, 16 }

implements the ConstantUnsignedArray31 trait for "limbs" of length 9 and 16, the two provided by the crate Fp256 and Fp480.

This issue can be solved by exporting the macro that implements the ConstantUnsignedArray31 trait like this

#[macro_export]
macro_rules! constant_unsigned_array31 { ($($N:expr),*) => { $(
...

and then allowing the end user to implement the trait themselves for their desired limbs.

Or, when using the fp31! macro to create a field of n limbs, use the constant_unsigned_array31! macro within the macro to implement the trait for [u32; $n].

[skeet70]

We don't have a need for this in our use cases, but we'd welcome a PR