update defmt deps to v0.3 (#56)

* bump defmt to v0.3

remove Cargo features which are no longer needed

* remove `timestamp!`

it's no longer mandatory and a monotonic counter does not add much value given that `defmt-rtt`
works by disabling interrupts so one won't see logs out of order anyways (which would the case for a
lock-free multi-channel `Logger`)

* use println! in most the examples

so that all examples print something when `DEFMT_LOG` is unset

* make overflow example terminate in less steps

also include an estimate of the stack pointer to show the stack grows downwards towards the RAM
boundary

src/lib.rs

@@ -1,8 +1,7 @@
 #![no_std]
 
-use core::sync::atomic::{AtomicUsize, Ordering};
-
 use defmt_rtt as _; // global logger
+
 // TODO(5) adjust HAL import
 // use some_hal as _; // memory layout
 
@@ -15,14 +14,6 @@ fn panic() -> ! {
     cortex_m::asm::udf()
 }
 
-static COUNT: AtomicUsize = AtomicUsize::new(0);
-defmt::timestamp!("{=usize}", {
-    // NOTE(no-CAS) `timestamps` runs with interrupts disabled
-    let n = COUNT.load(Ordering::Relaxed);
-    COUNT.store(n + 1, Ordering::Relaxed);
-    n
-});
-
 /// Terminates the application and makes `probe-run` exit with exit-code = 0
 pub fn exit() -> ! {
     loop {