This makes rtt output work right when using `cargo run` in release mode.
Debug was already enabled for release builds in some of the examples but
not all.
- probe-run screwed up the last release 2 weeks ago and it's still not fixed (issue 391). Doesn't look well maintained.
- Even when it's not broken, it lags behind probe-rs-cli in new chips support because it's slow in updating probe-rs.
This introduces a `Pender` struct with enum cases for thread-mode, interrupt-mode and
custom callback executors. This avoids calls through function pointers when using only
the thread or interrupt executors. Faster, and friendlier to `cargo-call-stack`.
`embassy-executor` now has `arch-xxx` Cargo features to select the arch and to enable
the builtin executors (thread and interrupt).
