For 20w14a

Adds hierarchical block decay. Slows the actual block to water/air clearing process, but makes gravel, sand, clay, and moss become renewable resources.

New Features:

• Edge blocks decay through the decay hierarchy before disappearing.
• Blocks decay upstream in stream beds, if the downstream block is the next decay target.
• Cobblestone and Stone Bricks grow moss near water.
  
  

Decay hierarchy:

• Cobblestone -> Mossy Cobblestone -> Gravel
• Stone Bricks -> Mossy Stone Bricks -> Gravel
• Grass Path -> Grass -> Dirt -> Coarse Dirt -> Gravel
• Gravel -> Sand -> Clay(rarely) -> Air/Water.