cargo: forward "unproven" feature dependency

[lucab]

This forwards the "unproven" feature from this crate to the
corresponding feature on dependencies.

[chrysn]

Building this with --features chip-efm32gg --no-default-features, I get

error[E0405]: cannot find trait `InputPin` in module `digital`
  --> src/gpio.rs:45:26
   |
45 |     type Input: digital::InputPin;
   |                          ^^^^^^^^ not found in `digital`

Was this maybe tested with an increased embedded-hal version? If so, that should be bumped in the PR.

[lucab]

@chrysn thanks for the feedback! No, there seems to be more places that are missing the #[cfg(feature = "unproven")] annotation, so the codebase needs to be massaged further.

Anyway, this PR got stale in the meanwhile and embedded-hal is dropping the whole unproven feature, see rust-embedded/embedded-hal#192.

I'd suggest to abandon this PR, and instead:

• bump the embedded-hal dependency to 0.2.3
• drop all the not(feature = "unproven") annotations, i.e. always require unproven
• switch to digital v2/fallible traits