Added get_name_from_content_id

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What added

Function get_content_id was added here: #13

To be decided

As content ids get generated when you ask for one there should not be id available for name before using minetest.get_content_id(name).
This makes minetest.get_name_from_content_id(cid) generate missing ids when called with unknown content id.

Should minetest.get_name_from_content_id(cid) fail immediately if called with unknown id?
Also is content id 0 based or 1 based, probably 0 based (also used in original commit) is correct?

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Ping @BuckarooBanzay as you've made original changes. Any ideas for questions stated?

Also fail / not fail with unknown things: mineunit is testing framework so excessive error/failure avoidance is of course bad thing, I think question is more about which way follows engine implementation better.

[BuckarooBanzay]

Should minetest.get_name_from_content_id(cid) fail immediately if called with unknown id?

Fail immediately would match with the current engine behavior

Also is content id 0 based or 1 based, probably 0 based (also used in original commit) is correct?

Not sure about that one, there are also some magic numbers (126 for air for example) but i'm not sure if/how they are translated into the game 🤷
IMO: shouldn't matter, no sane person would compare the content-id's with hardcoded values (i hope)

[S-S-X]

Cleaned up, minetest.get_name_from_content_id(cid) will crash if called before minetest.get_content_id(name).
This is probably fine and content ids can be generated on the fly while adding VoxelManip functionality and implementing VoxelArea #34, #35 and voxelmanip.lua

I think this is a lot more useful after mentioned things are done, currently seems to work like before without affecting behavior in any way.

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Bit deeper these things might actually matter, see comment #34 (comment)