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/bin/panic.rs

@@ -5,7 +5,7 @@ use {{crate_name}} as _; // global logger + panicking-behavior + memory layout
 
 #[cortex_m_rt::entry]
 fn main() -> ! {
-    defmt::info!("main");
+    defmt::println!("main");
 
     defmt::panic!()
 }