complete the FEC and "linear algebra" tests

• komodo::linalg::Matrix::random is tested
• komodo::linalg::Matrix::inverse is tested on more matrix sizes, from 1 to 20 random matrices
• komodo::field tests have been double-checked
• pure "recoding" tests from komodo::fec have been double-checked
• end_to_end and end_to_end_with_recoding now runs for k \in [3, 5] and \rho \in [\frac{1}{2}, \frac{1}{3}] with n = \lfloor \frac{k}{\rho} \rfloor
• all "k among n + t" combinations are tested with try_all_decoding_combinations, possibly with some removals in case recoding is involved with is_inside

Important
on my machine, make test goes from less than 8sec on latest main to around 40sec with this MR

src/fec.rs

@@ -304,9 +304,11 @@ mod tests {
         let mut shards = shards.unwrap();
         shards.push(shards[2].recode_with(to_curve(7), &shards[4], to_curve(6)));
         shards.push(shards[1].recode_with(to_curve(5), &shards[3], to_curve(4)));
+        shards.push(shards[n].recode_with(to_curve(3), &shards[n + 1], to_curve(8)));
 
         // n     = (2, 4)
         // n + 1 = (1, 3)
+        // n + 2 = (n, n + 1) = ((2, 4), (1, 3))
 
         try_all_decoding_combinations(
             data,
@@ -314,7 +316,11 @@ mod tests {
             k,
             &test_case,
             None,
-            vec![vec![2, 4, n], vec![1, 3, (n + 1)]],
+            vec![
+                vec![2, 4, n],
+                vec![1, 3, (n + 1)],
+                vec![n, (n + 1), (n + 2)],
+            ],
         );
     }