Improve multithreading support

[Aeledfyr]

This PR:

• Replaces all uses of RefCell with Mutex, RwLock, or OnceCell's sync variant
• Replaces all uses of Rc with Arc
• Adds Send + Sync bounds to all traits so that their trait objects are thread-safe
• Since this added once_cell, I replaced the uses of lazy_static with once_cell
• Disables unused features in the image crate (this change is mostly unrelated to threading and I can remove it if necessary)

Some of these changes likely have some performance overhead; two potential improvements would be reworking heightmap caching so that it only needs to be updated once and can use once_cell, and sharing the lock in the RegionBuffer::for_each_chunk function rather than unlocking it and relocking it many times (in each chunk_location call).

Fixes #46.

[owengage]

Looks good! I'll try this in a few full world renders as an informal benchmark soon. I don't expect the locking to have much impact. :)

[owengage]

Quick renders of hermitcraft season 8's world download:

multithread: 
    8.847
    9.100
    8.900
    8.851

master:
    8.890
    8.936
    8.747
    8.840

standard deviations:
multithread support: 8.92 +- 0.1
master: 8.85 +- 0.07

No significant different here as far as I'm concerned. Thank you for the contribution. :)