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/bitfield.rs

@@ -7,7 +7,7 @@ use {{crate_name}} as _; // global logger + panicking-behavior + memory layout
 fn main() -> ! {
     // value of the FREQUENCY register (nRF52840 device; RADIO peripheral)
     let frequency: u32 = 276;
-    defmt::debug!("FREQUENCY: {0=0..7}, MAP: {0=8..9}", frequency);
+    defmt::println!("FREQUENCY: {0=0..7}, MAP: {0=8..9}", frequency);
 
     {{crate_name}}::exit()
 }