Initial support for the Rust Embedded embedded-hal

[alistair23]

This is very initial support for the embedded_hal [1]. This allows general embedded Rust crates to be built on top of libtock-rs with minimal porting effort.

This currently just stubs out the support and implements the digital::OutputPin trait for GPIO pins.

1: https://docs.rs/embedded-hal/1.0.0/embedded_hal/index.html

[lschuermann]

I think adding support for the embedded HAL is a great addition. I met with some of the Embedded WG folks a couple days ago, and based on some informal conversations this seems like something that's in scope for the HAL too.

I'm not sure whether it would perhaps not be better to ship this as a separate crate. We can't extend libtock-rs' types in other crates, but we can introduce wrapper types that then implement the embedded HAL. I'm slightly skeptical of sprinkling these #[cfg( attributes throughout the codebase, as it can become quite hard to reason about the various interactions and interface compositions across permutations of features.

This seems like a good first step though.

[alistair23]

I'm not sure whether it would perhaps not be better to ship this as a separate crate. We can't extend libtock-rs' types in other crates, but we can introduce wrapper types that then implement the embedded HAL. I'm slightly skeptical of sprinkling these #[cfg( attributes throughout the conference, as it can become quite hard to reason about the various interactions and interface compositions across permutations of features.

A separate crate is probably the way to go. One of the issues we have is that libtock-rs uses types, which don't work very well.

For examples, for the SPI work I have this

#[cfg(feature = "rust_embedded")]
impl<S: Syscalls, C: Config> embedded_hal::spi::SpiDevice for SpiController<S, C> {
    fn transaction(
        &mut self,
        operations: &mut [embedded_hal::spi::Operation<'_, u8>],
    ) -> Result<(), Self::Error> {
...

Then following the standard libtock-rs style we have

pub mod spi_controller {
    use libtock_spi_controller as spi_controller;
    pub type SpiController = spi_controller::SpiController<super::runtime::TockSyscalls>;
}

But that results in errors like this when trying to actually use the type

93 |     let di = SPIInterface::new(SpiController, dc);
   |                                ^^^^^^^^^^^^^
   |
   = note: can't use a type alias as a constructor

Wrapper structs in a crate should help with that. I think this PR is still ok as a first step. Any thoughts on the way to setup the crate are appreciated

[alistair23]

Is this ready to merge now?