Add PinInput and PinOutput HAL

[soypat]

machine.Pin is not compileable by upstream go. To promote development of drivers which are usable by upstream Go I propose we add the following API.

// PinInput is hardware abstraction for a pin which receives a
// digital signal and reads it (high or low voltage).
type PinInput func() (level bool)

// PinOutput is hardware abstraction for a pin which outputs a
// digital signal (high or low voltage).
type PinOutput func(level bool)

[deadprogram]

What about #749 ?

[soypat]

I missed that @deadprogram. Will comment

[soypat]

@aykevl @ysoldak I've started work on the PR. Most of the infra is in place and the drivers I've worked on are good demonstrations of the proposal in action. I'll be on vacations for a few days so I might not be available for a week or so :)

[soypat]

I think around half of the work is done. On vacation this weekend so I'll get around sometime next week

[soypat]

baremetal tags are used for when the implementation must use the machine.Pin type to avoid user breakage.

[deadprogram]

I think we already use that buildtag in tinygo itself, no?

[soypat]

We use it for MCUs in TinyGo; is my understanding. So to let users use easystepper outside TinyGo I've split the implementation in baremetal and non-baremetal files protected with the baremetal tag.

[soypat]

OK! I just finished the brunt of the work. @aykevl @ysoldak @deadprogram @sago35 Tagging ready for review of the work done!