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

testsuite/Cargo.toml

@@ -15,25 +15,9 @@ harness = false
 
 [dependencies]
 {{project-name}} = { path = ".." }
-cortex-m = "0.7.1"
-cortex-m-rt = "0.6.12"
-defmt = "0.2.0"
-defmt-rtt = "0.2.0"
-defmt-test = "0.2.0"
-panic-probe = { version = "0.2.0", features = ["print-defmt"] }
-
-[features]
-# set logging levels here
-default = [
-  # in tests, enable all logs
-  "defmt-trace",
-  # "dependency-a/defmt-trace",
-]
-
-# do NOT modify these features
-defmt-default = []
-defmt-trace = []
-defmt-debug = []
-defmt-info = []
-defmt-warn = []
-defmt-error = []
+cortex-m = "0.7.3"
+cortex-m-rt = "0.7.0"
+defmt = "0.3.0"
+defmt-rtt = "0.3.0"
+defmt-test = "0.3.0"
+panic-probe = { version = "0.3.0", features = ["print-defmt"] }