The code using sprintf() did not fit into flash when `merge/um70:via`
was compiled with avr-gcc 5.4.0:
* The firmware is too large! 29756/28672 (1084 bytes over)
Replacing `sprintf(wpm_str, " %03d", current_wpm);` with custom
formatting code reduces the firmware size by 1504 bytes, which is enough
to make the `merge/um70:via` firmware fit:
* The firmware size is approaching the maximum - 28252/28672 (98%, 420 bytes free)
3aca3d3572