make code base clippy-clean

[zacchiro]

currently we do not pass "cargo clippy" due to the following errors (there are also warnings, but they are not blocking):

error: read amount is not handled
  --> src/backends/file_backend.rs:68:9
   |
68 |         self.file.read(res.as_mut())?;
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: use `Read::read_exact` instead, or handle partial reads
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_io_amount
   = note: `#[deny(clippy::unused_io_amount)]` on by default

error: written amount is not handled
  --> src/backends/file_backend.rs:79:9
   |
79 |         self.file.write(word.to_ne_bytes().as_ref())?;
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: use `Write::write_all` instead, or handle partial writes
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_io_amount

[zacchiro]

Edit: I've edited the snippt the clippy output above to the only two errors that matter for now, the rest are warnings that are not blocking for now.

Both outstanding errors are related to unhandled byte counts returned by read/write, which should be easy to fix.