alternate pin function enums

[burrbull]

No description provided.

[burrbull]

@richardeoin Is there other reason for new_unchecked except of possibility to release?

New API allows you to keep pins inside structure without advanced <PINS> generic (at least for simple cases). See:
https://github.com/stm32-rs/stm32f4xx-hal/blob/557b7f2de9963ef2cbe11b554d1eab7894b9a007/src/i2c.rs#L71-L74

[richardeoin]

The new_unchecked methods are also useful for using a peripheral without encoding its GPIOs in the type system. In some situations the code and effort required to state the type of all GPIOs is excessive, so the new_unchecked methods are a useful escape hatch. For example to setup the GPIOs in main(), and setup a peripheral in my_func() without passing GPIOs as arguments to my_func(). With this PR there is less need for the new_unchecked methods, but I would rather keep them available for now.

Overall this PR is quite a nice usability improvement. I would like to play with it some more, but in general looks good! :)

[burrbull]

rebased