refactor: Remove some Utils modules

[anmonteiro]

• JOSE supports OCaml 4.08+
• These functions are all available in 4.08

[codecov]

Codecov Report

Merging #65 (7afcb46) into master (226d4c9) will increase coverage by 0.61%.
Report is 1 commits behind head on master.
The diff coverage is 86.60%.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
+ Coverage   74.65%   75.26%   +0.61%     
==========================================
  Files           8        8              
  Lines         935      926       -9     
==========================================
- Hits          698      697       -1     
+ Misses        237      229       -8     

Files Changed	Coverage Δ
jose/Jwt.ml	79.41% <80.00%> (ø)
jose/Jwe.ml	84.12% <84.14%> (+1.22%)	⬆️
jose/Jwk.ml	66.46% <84.61%> (-0.14%)	⬇️
jose/Jws.ml	83.89% <92.59%> (-0.68%)	⬇️
jose/Header.ml	92.15% <100.00%> (ø)
jose/Jwks.ml	90.00% <100.00%> (ø)
jose/Utils.ml	89.65% <100.00%> (+17.10%)	⬆️

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[ulrikstrid]

I will review this a bit later today

[anmonteiro]

addressed review