Add instance() method to all NativeConverter implementations

[ExE-Boss]

Should have been in #243 and #275

Completed:

• [x] Add an instance() method to all NativeConverter implementations
• [x] Fix warnings in NativeManager
• [x] Autocast result of NativeManager.getNative and NativeManager.findConverter

[codecov-io]

Codecov Report

Merging #284 into master will decrease coverage by 0.06%. The diff coverage is 24.24%.

@@             Coverage Diff              @@
##             master     #284      +/-   ##
============================================
- Coverage     13.38%   13.32%   -0.07%     
- Complexity      679      680       +1     
============================================
  Files           403      404       +1     
  Lines         12078    12154      +76     
  Branches       1296     1297       +1     
============================================
+ Hits           1617     1619       +2     
- Misses        10362    10436      +74     
  Partials         99       99

Impacted Files	Coverage Δ	Complexity Δ
...orge/v17/wrapper/inventory/InventoryConverter.java	0% <0%> (ø)	0 <0> (ø)	:x:
...apper/mc/forge/v17/wrapper/item/ItemConverter.java	0% <0%> (ø)	0 <0> (ø)	:x:
...r/mc/forge/v17/wrapper/cuboid/CuboidConverter.java	0% <0%> (ø)	0 <0> (ø)	:x:
...r/mc/forge/v18/wrapper/entity/EntityConverter.java	0% <0%> (ø)	0 <0> (ø)	:x:
...rapper/mc/forge/v18/wrapper/CategoryConverter.java	0% <0%> (ø)	0 <0> (ø)	:x:
.../forge/v17/wrapper/block/world/WorldConverter.java	0% <0%> (ø)	0 <0> (ø)	:x:
...er/mc/forge/v18/wrapper/assets/AssetConverter.java	27.27% <0%> (ø)	3 <0> (ø)	:x:
...apper/mc/forge/v17/wrapper/DirectionConverter.java	81.81% <0%> (-3.9%)	15 <0> (ø)
...orge/v18/wrapper/inventory/InventoryConverter.java	0% <0%> (ø)	0 <0> (ø)	:x:
.../forge/v18/wrapper/block/world/WorldConverter.java	0% <0%> (ø)	0 <0> (ø)	:x:
... and 16 more

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