Use chunks_exact for performance in diff method

[shssoichiro]

This is part of a series of commits authored by @maj160 to improve performance of rav1e.

This commit results in an overall performance improvement of about 2% at speed 2 10-bit:

Benchmark 1: ~/Downloads/rav1e_master -s 2 --quantizer 64 ~/xiph-media-files/objective-1-fast-10bit/speed_bag_640x360_60f.y4m -y -o /dev/null
  Time (mean ± σ):     43.695 s ±  0.151 s    [User: 43.684 s, System: 0.180 s]
  Range (min … max):   43.518 s … 44.033 s    10 runs

Benchmark 2: ~/Downloads/rav1e_mod -s 2 --quantizer 64 ~/xiph-media-files/objective-1-fast-10bit/speed_bag_640x360_60f.y4m -y -o /dev/null
  Time (mean ± σ):     42.808 s ±  0.081 s    [User: 42.828 s, System: 0.170 s]
  Range (min … max):   42.702 s … 42.961 s    10 runs

[codecov-commenter]

Codecov Report

Base: 86.94% // Head: 86.77% // Decreases project coverage by -0.17% ⚠️

Coverage data is based on head (e4a5aab) compared to base (1c8c772).
Patch coverage: 69.72% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3092      +/-   ##
==========================================
- Coverage   86.94%   86.77%   -0.18%     
==========================================
  Files          83       83              
  Lines       33092    33105      +13     
==========================================
- Hits        28773    28726      -47     
- Misses       4319     4379      +60     

Impacted Files	Coverage Δ
src/rdo.rs	85.20% <ø> (-0.47%)	⬇️
src/predict.rs	79.36% <46.47%> (-2.14%)	⬇️
src/asm/x86/predict.rs	43.29% <55.26%> (+2.03%)	⬆️
src/header.rs	69.35% <66.66%> (+0.33%)	⬆️
src/asm/x86/ec.rs	100.00% <100.00%> (ø)
src/dist.rs	98.95% <100.00%> (-0.04%)	⬇️
src/ec.rs	83.51% <100.00%> (ø)
src/encoder.rs	86.95% <100.00%> (-0.18%)	⬇️
src/frame/plane.rs	100.00% <100.00%> (ø)
src/partition.rs	76.01% <100.00%> (-1.73%)	⬇️
... and 7 more

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