Per-world content IDs

[TheElectronWill]

Given that the internal ids could change between different servers or even between different runs of the same server, the ids of blocks, entities, etc. should be saved in the world's files.

Let's make a file for each world that maps typenames (universal) to saved ids (specific to the world).
On world load, two maps will be constructed from this file:

Map[saved id -> internal id]: Index[Int]
Map[internal id -> saved id]: Index[Int]

This is a part of #17.