Rename `driver_number` to `driver_num`

[jrvanwhy]

In https://github.com/tock/libtock-rs/pull/373, we noticed that this repository is split between using driver_number/DRIVER_NUMBER and driver_num/DRIVER_NUM. We decided to prefer driver_num/DRIVER_NUM for consistency with the main Tock repository.

We should replace all instances of driver_number/DRIVER_NUMBER in this repository with driver_num/DRIVER_NUM.

[alexandruradovici]

I think #373 solves this.

[jrvanwhy]

I think #373 solves this.

#373 replaced DRIVER_ID with DRIVER_NUM, but there's still a mix of DRIVER_NUM and DRIVER_NUMBER.

[bradjc]

Looks finished?

$ cd git/libtock-rs
$ git grep driver_number | cat
unittest/src/fake/syscalls/allow_ro_impl_tests.rs:fn too_large_driver_number() {
unittest/src/fake/syscalls/allow_rw_impl_tests.rs:fn too_large_driver_number() {

$ git grep DRIVER_NUMBER | wc -l
       0

[jrvanwhy]

Looks finished to me too.