What API to expose to TinyGo users when initializing drivers which receive pin HAL

[soypat]

This is meant to be a long running discussion. I've created it to link in an upcoming PR so future users can find it.

As a result of the following listed PRs I've chosen to create a discussion to focus a dialogue in a single place.

• refactor: add drivers.Pin interface drivers#749
• Add PinInput and PinOutput HAL drivers#753

The topic of discussion centers around what type should a New driver constructor receive for pin HAL.

Two main candidates are:

• An interface type
• A function type

// function type
type PinOutput func(level bool)

// interface type
type PinOutput interface {
   Set(level bool)
}

// driver constructor. TinyGo user facing API.
func NewDriver(csPin drivers.PinOutput) *Driver {
     return &Driver{cs: csPin}
}

// When using with interface type
d := NewDriver(machine.GPIO1)

// When using with function type
d := NewDriver(machine.GPIO1.Set)

Main data points:

• Interface types are more familiar to Go developers than function types
• Interface types can be at least 25 times slower than function types on some microcontrollers. See Add PinInput and PinOutput HAL drivers#753 (comment). Slowdown depends on number of interface implementations compiled.

Other arguments:

• Functions are easier to define than interfaces
• Functions are conceptually simpler than interface types