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

@@ -306,7 +306,7 @@ mod tests {
             vec![1, 3],       // = (n + 1)
             vec![n, (n + 1)], // = (n + 2) = ((2, 4), (1, 3))
             vec![0],          // = (n + 3) = (0)
-            vec![0],          // = (n + 4) = (0)
+            vec![(n + 3)],    // = (n + 4) = (0)
         ];
         let should_not_be_decodable = vec![
             vec![2, 4, n],