Saving: Added Region Save Mode

A very scalable system. Sectors (8x8x8 blocks) are stored 4x4x4 in headerless
files, 2 bytes per block. Position data is not stored, but inferred based on
the byte offset within the file.

Whole regions (32x32x32 blocks) are read at once when world._show_sector asks.
Sectors can be written to individually, though without a dirty_sectors system (TODO)
it currently writes all sectors in memory to disk during a save.