implement projection for InplaceableThunk

[mzgubic]

Closes #538

[codecov-commenter]

Codecov Report

Merging #541 (ed372aa) into main (3e11e62) will decrease coverage by 6.65%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #541      +/-   ##
==========================================
- Coverage   93.41%   86.75%   -6.66%     
==========================================
  Files          15       14       -1     
  Lines         865      687     -178     
==========================================
- Hits          808      596     -212     
- Misses         57       91      +34     

Impacted Files	Coverage Δ
src/projection.jl	88.55% <100.00%> (-8.75%)	⬇️
src/config.jl	0.00% <0.00%> (-100.00%)	⬇️
src/ignore_derivatives.jl	50.00% <0.00%> (-25.00%)	⬇️
src/tangent_types/tangent.jl	71.29% <0.00%> (-14.22%)	⬇️
src/tangent_types/thunks.jl	86.04% <0.00%> (-9.41%)	⬇️
src/tangent_types/abstract_zero.jl	90.47% <0.00%> (-4.98%)	⬇️
src/accumulation.jl	92.85% <0.00%> (-4.37%)	⬇️
src/rule_definition_tools.jl	93.27% <0.00%> (-3.00%)	⬇️
src/rules.jl	88.88% <0.00%> (-2.03%)	⬇️
... and 4 more

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[mcabbott]

This is fine by me. But I think @oxinabox objected to this the first time around.