interp/remap: improve support for remapping builtin codes

The G_REMAPPABLE and M_REMAPPABLE macros were getting a bit baroque.
Replace these macros by a bitset lookups, add init code for same.
interp/remap: make G28/G28.1/G30/G30.1 remappable
interp/remap: add G28/G28.1/G30/G30.1 example to remap/getting-started
Merge remote-tracking branch 'mah/remap-g30'