cleanup and documentation (!45)

> **Note**  
> this MR is best reviewed commit by commit, it's hopefully clear enough.

> **Note**  
> waiting for !47 to land

## changelog
- add missing format to the `KomodoError`s
- rename `fec::Shard::bytes` to `fec::Shard::data` because these are no _bytes_
- rename the `blocks: Vec<Shard<E>>` argument of `fec::decode` to `shards`
- refactor the tests of `fec` and `lib.rs`
- remove the `batch_verify` function entirely as it's pretty useless
- simplify the `linalg::Matrix::random` function
- add tests for the function in the `setup` module
- add documentation where it was missing