fix typos and improve algebra/semi_avid tests (!189)

mainly fixes a few typos and improves `algebra` and `semi_avid` tests

## changelog
- fix some typos and notes in the documentation
- test more cases for `algebra::split_data_into_field_elements` and `algebra::merge_elements_into_bytes`, more data lengths and more modulus respectively
- remove a useless `::<Vec<_>>` on a `collect` in `fec`
- `semi_avid::tests`
  - refactor `bls12-381` into a constant
  - write an "_attack_" function to alter a particular block, `attack<F, G>(block: Block<F, G>, c: usize, base: u128, pow: u64) -> Block<F, G>`
  - pass a list of attacks, i.e. `attacks: Vec<(usize, usize, u128, u64)>`, to `verify_with_errors_template`, use the same as the previous hardcoded one
  - pass a list of "_recodings_", i.e. `recodings: Vec<Vec<usize>>`, to `verify_recoding_template` and assert the number of blocks, e.g. `vec![vec![2, 3], vec![3, 5]]` means that recoding 2 and 3 together should verify, and same with 3 and 5, and also ...