fix overflow issue in interpolation

[PSeitz]

use saturating_sub and saturating_add to cover edge cases with values close to u64::MAX or 0 in combination with imprecise computation

fixes #1479

[codecov-commenter]

Codecov Report

Merging #1480 (3984caf) into main (513f682) will decrease coverage by 0.00%.
The diff coverage is 83.20%.

@@            Coverage Diff             @@
##             main    #1480      +/-   ##
==========================================
- Coverage   94.08%   94.07%   -0.01%     
==========================================
  Files         239      239              
  Lines       44768    44800      +32     
==========================================
+ Hits        42120    42146      +26     
- Misses       2648     2654       +6     

Impacted Files	Coverage Δ
fastfield_codecs/src/bitpacked.rs	100.00% <ø> (ø)
fastfield_codecs/src/main.rs	1.01% <0.00%> (ø)
src/fastfield/mod.rs	94.76% <ø> (-0.01%)	⬇️
src/fastfield/reader.rs	88.67% <42.85%> (+0.94%)	⬆️
src/fastfield/serializer/mod.rs	89.26% <82.75%> (-1.03%)	⬇️
fastfield_codecs/src/lib.rs	97.22% <95.83%> (+0.62%)	⬆️
fastfield_codecs/src/blockwise_linear.rs	100.00% <100.00%> (ø)
fastfield_codecs/src/linear.rs	99.58% <100.00%> (ø)
src/fastfield/gcd.rs	100.00% <100.00%> (ø)
src/fastfield/writer.rs	91.18% <100.00%> (ø)
... and 5 more

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[fulmicoton]

@PSeitz approved... but let's switch to integer arithmetics.