Add support for Ratio<i128> as an underlying storage type.

[iliekturtles]

Another old PR that has been lying around forever. Adds Ratio<i128> as an underlying storage type.

Would anyone find this type useful?

Does anyone think this type should not be added?

[codecov-commenter]

Codecov Report

Merging #301 (c41ae8e) into master (66d3e85) will decrease coverage by 0.10%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #301      +/-   ##
==========================================
- Coverage   97.30%   97.20%   -0.11%     
==========================================
  Files          59       59              
  Lines        2898     2898              
==========================================
- Hits         2820     2817       -3     
- Misses         78       81       +3     

Impacted Files	Coverage Δ
src/lib.rs	25.00% <ø> (ø)
src/si/thermodynamic_temperature.rs	86.95% <ø> (ø)
src/tests/quantity.rs	98.93% <ø> (-1.07%)	⬇️
src/tests/system.rs	100.00% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 66d3e85...c41ae8e. Read the comment docs.