include module path info in the output

[japaric]

depends on knurling-rs/defmt#151
Example input:

// src/bin/hello.rs
fn main() -> ! {
    defmt::info!("program start");
    // ..
}

// src/hal.rs
fn init() {
    defmt::debug!("initializing the HAL");
    // ..
}

Example output:

0.000000 INFO  [my_app::hello::main] program start
└─ src/bin/hello.rs:8
0.000001 DEBUG [my_app::hal::init] initializing the HAL
└─ src/hal.rs:8

[japaric]

done in #57