Express fold nodes using accumulator nodes

[knoepfel]

This PR replaces the caching mechanism for fold results with a similar mechanism as used in #118.

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Resolves #359

[codecov]

Codecov Report

❌ Patch coverage is 94.73684% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
phlex/core/detail/accumulator_node.hpp	94.73%	1 Missing and 4 partials ⚠️

@@            Coverage Diff             @@
##             main     #540      +/-   ##
==========================================
- Coverage   86.74%   85.80%   -0.95%     
==========================================
  Files         142      143       +1     
  Lines        5033     3755    -1278     
  Branches      631      644      +13     
==========================================
- Hits         4366     3222    -1144     
+ Misses        452      313     -139     
- Partials      215      220       +5     

Flag	Coverage Δ
unittests	85.80% <94.73%> (-0.95%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
phlex/core/fold/send.hpp	100.00% <ø> (ø)
phlex/core/detail/accumulator_node.hpp	94.73% <94.73%> (ø)

... and 113 files with indirect coverage changes

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