Merge #390

390: Spawn and schedule from anywhere r=AfoHT a=korken89

This PR moves RTIC to the spawn and schedule from anywhere syntax.

Notable changes:
 * We do no longer support non-`Send` types.
 * Some extra code is generated as any task may spawn/schedule any task. However Rust/LLVM does a great job optimizing away non used instantiations (no real code-size difference observed).
 * Worst case priority inversion has increased, but it is now predictable.

Upsides:
 * With this we should be able to support async/await.
 * RTIC tasks can now be callbacks (spawned and scheduled).
 * RTIC tasks can be stored.

Needs the following PR to land first: https://github.com/rtic-rs/rtic-syntax/pull/34

The following now works:

```rust
#[rtic::app(device = lm3s6965, monotonic = rtic::cyccnt::CYCCNT)]
mod app {
    #[init]
    fn init(mut cx: init::Context) -> init::LateResources {
        // Init stuff...

        // New spawn syntax
        foo::spawn().unwrap();

        // New schedule syntax
        bar::schedule(now + 4_000_000.cycles()).unwrap();

        init::LateResources {}
    }

    #[task]
    fn foo(_: foo::Context) {}

    #[task]
    fn bar(_: bar::Context) {}

    extern "C" {
        fn SSI0();
    }
}

```

Co-authored-by: Per Lindgren <per.lindgren@ltu.se>
Co-authored-by: Emil Fresk <emil.fresk@gmail.com>

1 files changed, 4 insertions, 4 deletions

diff --git a/examples/periodic.rs b/examples/periodic.rs
index d3aedd3..95cd145 100644
--- a/examples/periodic.rs
+++ b/examples/periodic.rs
@@ -15,21 +15,21 @@ const PERIOD: u32 = 8_000_000;
 #[rtic::app(device = lm3s6965, monotonic = rtic::cyccnt::CYCCNT)]
 mod app {
 
-    #[init(schedule = [foo])]
+    #[init]
     fn init(cx: init::Context) -> init::LateResources {
         // omitted: initialization of `CYCCNT`
 
-        cx.schedule.foo(cx.start + PERIOD.cycles()).unwrap();
+        foo::schedule(cx.start + PERIOD.cycles()).unwrap();
 
         init::LateResources {}
     }
 
-    #[task(schedule = [foo])]
+    #[task]
     fn foo(cx: foo::Context) {
         let now = Instant::now();
         hprintln!("foo(scheduled = {:?}, now = {:?})", cx.scheduled, now).unwrap();
 
-        cx.schedule.foo(cx.scheduled + PERIOD.cycles()).unwrap();
+        foo::schedule(cx.scheduled + PERIOD.cycles()).unwrap();
     }
 
     // RTIC requires that unused interrupts are declared in an extern block when