py/emitbc: Make all emit_write_bytecode_* funcs take a stack_adj arg.

This factoring of code gives significant code-size savings:

   bare-arm:  -456 -0.682%
minimal x86:  -844 -0.547%
   unix x64:  -472 -0.095%
unix nanbox: -1348 -0.303%
      stm32:  -472 -0.130% PYBV10
     cc3200:  -448 -0.242%
    esp8266:  -708 -0.108%
      esp32:  -400 -0.036% GENERIC
        nrf:  -520 -0.356% pca10040
       samd:  -456 -0.448% ADAFRUIT_ITSYBITSY_M4_EXPRESS