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, 5 insertions, 5 deletions

diff --git a/examples/baseline.rs b/examples/baseline.rs
index 3ab40dbb..0b7e3ea0 100644
--- a/examples/baseline.rs
+++ b/examples/baseline.rs
@@ -12,19 +12,19 @@ use panic_semihosting as _;
 // NOTE: does NOT properly work on QEMU
 #[rtic::app(device = lm3s6965, monotonic = rtic::cyccnt::CYCCNT)]
 mod app {
-    #[init(spawn = [foo])]
+    #[init]
     fn init(cx: init::Context) -> init::LateResources {
         // omitted: initialization of `CYCCNT`
 
         hprintln!("init(baseline = {:?})", cx.start).unwrap();
 
         // `foo` inherits the baseline of `init`: `Instant(0)`
-        cx.spawn.foo().unwrap();
+        foo::spawn().unwrap();
 
         init::LateResources {}
     }
 
-    #[task(schedule = [foo])]
+    #[task]
     fn foo(cx: foo::Context) {
         static mut ONCE: bool = true;
 
@@ -39,12 +39,12 @@ mod app {
         }
     }
 
-    #[task(binds = UART0, spawn = [foo])]
+    #[task(binds = UART0)]
     fn uart0(cx: uart0::Context) {
         hprintln!("UART0(baseline = {:?})", cx.start).unwrap();
 
         // `foo` inherits the baseline of `UART0`: its `start` time
-        cx.spawn.foo().unwrap();
+        foo::spawn().unwrap();
     }
 
     // RTIC requires that unused interrupts are declared in an extern block when