GPU-friendly truncation implementations

[lkdvos]

This is an attempt to get rid of the scalar-indexing oriented approach, and instead do more global operations.
Definitely still WIP, and on CPU there are definitely various optimizations that can be applied if needed.
I do wonder about the performance a bit, as I would actually expect that for a large number of sectors this might just be faster.

Some possible optimizations:

• for UniqueFusion, finding the nth value is simply partialsortperm(values, n; by, rev), avoiding the need to allocate the full permutation vector
• for CPU, cumsum + findlast can be replaced by a loop to avoid some intermediate allocations

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/factorizations/truncation.jl	54.80% <100.00%> (-33.13%)	⬇️

... and 31 files with indirect coverage changes

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