.) Use already existing inverse_millimeters instead of /
block->millimeters.
.) Prevent overflow during acceleration calculation by checking if float
is necessary. Idea modified from Sailfish.
.) Save two uint32_t or even float multiplications by checking if
step[AXIS] has steps and if max acceleration is lower than accel. If
not, there is no need to check this axis.

…and apply const to some method parameters

Use to sync the planner after an interrupted move (when not overriding
the logical position).

Use lx, ly, lz for “logical” positions

- `per_sq_second` => `per_s2`
- `per_sqr_second` => `per_s2`
- `axis_steps_per_sqr_second` => `max_acceleration_steps_per_s2`

This change actually does increase the binary size by about 12 bytes,
but how does it affect performance?

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

Following up on #3231

Keep "astyled" reformatting

With the introduction of the #if ENABLED(…)

reads better than

Apply `ENABLED` / `DISABLED` macros to planner files.

With these changes the output of `M503 S0` is all you need to restore
the EEPROM. Building on this it is straightforward to save and restore
the EEPROM state using the SD card or external GCode file.

- Added `M145` to set “heatup states” for the LCD menu
- Added `M420` to toggle Mesh Bed Leveling
- Added `M421` to set a single Mesh coordinate
- Extended `Config_PrintSettings` with added M codes
- Cleaned up some comments here and there

- Add some documentation to planner and stepper headers
- Patch up RAMBO pins with undefs
- Add `sync_plan_position` inline to set current XYZE
- Swap indices in `extruder_offset` to fix initialization values

- Use named axis indexes, `X_AXIS` etc.
- Replace `block.steps_A` with block.steps[A]`
- Replace `A_segment_time` with `segment_time[A]`
- Add `A_AXIS`, `B_AXIS` for `COREXY` axes
- Conditional compile based on `EXTRUDERS`
- Add BLOCK_MOD macro for planner block indexes
- Apply coding standards to `planner.h` and `planner.cpp`
- Small optimizations of planner code
- Update `stepper.cpp` for new `block` struct
- Replace `memcpy` with loops, let the compiler unroll them
- Make `movesplanned` into an inline function

Added option to set Travel Acceleration (non printing moves).
The M204 options was a non sense (S for printing moves and T for retract
moves).

It has been changed to:
P = Printing moves
R = Retract only (no X, Y, Z) moves
T = Travel (non
printing) moves

I will add this info o G-Code wiki in reprap.org. I also advise to put
this info in Marlin next version changelog.