Re-export pin types from gpio

Now pins can be accessed using, for example, `crate::gpio::PA1` instead
of `crate::gpio::gpioa::PA1`. The names are already unambigous, and not
only can this make all imports slighly more convenient, it can also help
cut down a lot on use statements required for macro-generated code.

Author: hannobraun

src/gpio.rs

@@ -731,6 +731,10 @@ macro_rules! gpio {
                 }
             )+
         }
+
+        pub use $gpiox::{
+            $($PXi,)*
+        };
     }
 }