Secret preparation modifier for resizing

[gdalle]

First step of the solution to #519, creates the syntax but doesn't provide any optimized implementation.

DI source

• Add secret functions prepare!_operator(f, other_prep, backend, x) which have the same output as prepare_operator(f, backend, x) but possibly modify the other_prep object to save memory, e.g. by resizing it if it came from an x2 with a different size.

DIT source

• Add a modified preparation in the correctness tests. Right now it's not modified from an array of different size, just from the same array (it's a bit of a hassle to check that if we prepare from a smaller or larger array, the contexts also apply).

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.70%. Comparing base (3f29b61) to head (cc4bc4e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #521      +/-   ##
==========================================
- Coverage   98.71%   98.70%   -0.01%     
==========================================
  Files         105      106       +1     
  Lines        4525     4571      +46     
==========================================
+ Hits         4467     4512      +45     
- Misses         58       59       +1     

Flag	Coverage Δ
DI	98.87% <100.00%> (-0.03%)	⬇️
DIT	98.35% <100.00%> (+0.03%)	⬆️

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