New Drop interface, compat for Project Red, Chisel

Instead of a single class, now the drops use a simple interface, of
which the old class in only one possible implementation. RareDrop should
be useful for well, rarer drops from ores. Already implemented some
tests, but the numbers will need a lot of work.

Chisel and Project Red compat are for marble, limestone and basalt. You
can now chisel any of them into any others, including the Project Red
forms.