Makes the digital::v2::InputPin trait proven

See #41 for more details about this trait.

Removes the feature flag from v2 implementation

Keeps it on v1 because it would not make sense to prove a deprecated
feature.

Author: nils-van-zuijlen

src/digital/v1_compat.rs

@@ -108,6 +108,8 @@ where
 
 /// Wrapper to allow fallible `v2::InputPin` traits to be converted to `v1::InputPin` traits
 /// where errors will panic.
+///
+/// Available behind `"unproven"` feature because `v1::InputPin` is
 #[cfg(feature = "unproven")]
 pub struct OldInputPin<T> {
     pin: T,
@@ -139,6 +141,8 @@ where
 
 /// Implementation of `v1::InputPin` trait for `v2::InputPin` fallible pins
 /// where errors will panic.
+///
+/// Available behind `"unproven"` feature because `v1::InputPin` is
 #[cfg(feature = "unproven")]
 #[allow(deprecated)]
 impl <T, E> v1::InputPin for OldInputPin<T>

src/digital/v2.rs

@@ -124,9 +124,6 @@ pub mod toggleable {
 }
 
 /// Single digital input pin
-///
-/// *This trait is available if embedded-hal is built with the `"unproven"` feature.*
-#[cfg(feature = "unproven")]
 pub trait InputPin {
     /// Error type
     type Error;

src/digital/v2_compat.rs

@@ -76,6 +76,8 @@ where
 impl<T> v2::toggleable::Default for T where T: v1::toggleable::Default {}
 
 /// Implementation of fallible `v2::InputPin` for `v1::InputPin` digital traits
+///
+/// Available behind `"unproven"` feature because `v1::InputPin` is
 #[cfg(feature = "unproven")]
 #[allow(deprecated)]
 impl <T> v2::InputPin for T