run FEC decoding test in a loop

changelog

• refactor k and n into variables
• run the decoding FEC test in a loop from (k - 1) \rho + 1 to k \rho, where \rho is the curve number of bytes per scalar field element, e.g. 31 for BLS-12-381
• add a NOTE about having too few bytes and elements, i.e. below (k - 1) \rho + 1
• add a FIXME about having too many bytes and elements, i.e. above k \rho

src/fec.rs

@@ -339,11 +339,13 @@ mod tests {
 
     #[test]
     fn decoding() {
+        let (k, n) = (3, 5);
+
         let modulus_byte_size = <Bls12_381 as Pairing>::ScalarField::MODULUS_BIT_SIZE as usize / 8;
-        // NOTE: starting at `modulus_byte_size * 2 + 1` to include at least _k = 3_ elements
-        // FIXME: stopping at 3 elements, more yields crashes
-        for b in (modulus_byte_size * 2 + 1)..=(modulus_byte_size * 3) {
-            decoding_template::<Bls12_381>(&BYTES[..b], 3, 5);
+        // NOTE: starting at `modulus_byte_size * (k - 1) + 1` to include at least _k_ elements
+        // FIXME: stopping at k elements, more yields crashes
+        for b in (modulus_byte_size * (k - 1) + 1)..=(modulus_byte_size * k) {
+            decoding_template::<Bls12_381>(&BYTES[..b], k, n);
         }
     }