Access previous/next step during compile

[freenerd]

Currently, the compile(step) function only has the current step as input. There are cases that we can only solve by allowing access to the previous/next step:

• Mode changes Handling mode changes #18
• "Stay on ..." instruction "Stay on" when continuing on the same road #41
• Combined instructions One Step at a Time vs Combined Steps #15

[mkv123]

Related to starting work on adding support for mode changes (#18), I will need to tackle this one to some extent as well. I think the easiest way is to at least initially keep the current compile(language, step, options) as is, and instead add something like compileLeg(language, leg, options) which then internally uses the single step compile function. Then to the single step compile function, I would add the needed options to make it aware when modes change etc, which the compileLeg would populate. The return value of compileLeg would be an array of the instructions.

This would keep backwards compatibility for compile() and reduce the amount of changes to that function.

What do you think of this approach?