As noted by https://github.com/MarlinFirmware/Marlin/pull/4971#issuecomment-252503295

・Fix a comment about timer

.) long to int: Extruder stalls at 10kHz / 20kHz step limits with long.
.) Take the delta_adv_steps calculation out of the step_loops loop. Wasted calculation performance if done inside.
.) >> 2 replaced by 3: Is divide by 8. Reason: Timer 0 runs at 16/8=2MHz, Timer 1 at 16/64=0.25MHz. ==> 2/0.25=8.

Probably needed for long straight lines when using high microstepping

Remove a setting proccess of E direction in advance_isr()

Also the extruder stepper ISR has to keep an eye on step_loops count.

- Search all symbols and apply prefixes where needed
- Encapsulate some private methods
- Inline some setters
- Make `microstep_mode` a public method