[WIP] Tree generation

[caelunshun]

• Implement a Schematic type for storing structures and writing parts of them which overlap with a chunk to the chunk
• Implement tree schematic generators
• Implement a tree finisher, with tree types/density based on biomes

[codecov]

Codecov Report

Merging #138 into develop will decrease coverage by 0.24%.
The diff coverage is 53.69%.

@@             Coverage Diff             @@
##           develop     #138      +/-   ##
===========================================
- Coverage    60.69%   60.45%   -0.25%     
===========================================
  Files           77       82       +5     
  Lines         9208     9516     +308     
===========================================
+ Hits          5589     5753     +164     
- Misses        3619     3763     +144

Impacted Files	Coverage Δ
server/src/worldgen/finishers/single.rs	86.36% <ø> (ø)	⬆️
server/src/worldgen/finishers/snow.rs	100% <ø> (ø)	⬆️
server/src/worldgen/finishers/clumped.rs	96% <ø> (ø)	⬆️
server/src/worldgen/finishers/tree/ty.rs	0% <0%> (ø)
server/src/worldgen/finishers/tree/schematic.rs	0% <0%> (ø)
server/src/worldgen/mod.rs	97.88% <100%> (-0.02%)	⬇️
server/src/worldgen/finishers/tree/mod.rs	55% <55%> (ø)
server/src/worldgen/schematic.rs	69.34% <69.34%> (ø)
server/src/worldgen/presence.rs	94.87% <94.87%> (ø)
... and 4 more

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[caelunshun]

I noticed a severe performance impact from adding this, and noticed a rather peculiar cause when investigating:

bitvec is for some reason the main overhead for worldgen, which seems somewhat odd...