Cannot run the tests in continuous integration

[therealprof]

Some tests fail both on stable and on nightly compilers which means we cannot use CI on this crate at the moment, cf #23 and particularly: https://github.com/rust-embedded/nb/runs/801586296?check_suite_focus=true

On nightly we get:

---- src/lib.rs -  (line 293) stdout ----
error[E0599]: no method named `resume` found for generator `[generator@src/lib.rs:15:22: 29:6 _]` in the current scope
  --> src/lib.rs:332:16
   |
40 |         blinky.resume();
   |                ^^^^^^ method not found in `[generator@src/lib.rs:15:22: 29:6 _]`

error[E0599]: no method named `resume` found for generator `[generator@src/lib.rs:31:24: 36:6 _]` in the current scope
  --> src/lib.rs:333:18
   |
41 |         loopback.resume();
   |                  ^^^^^^ method not found in `[generator@src/lib.rs:31:24: 36:6 _]`

error: aborting due to 2 previous errors

On stable we receive tons of errors about unstable #[feature] attributes.