Implement parser for hexadecimal floating numbers

[st-pasha]

Closes #2316

[codecov-io]

Codecov Report

Merging #2359 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2359      +/-   ##
==========================================
+ Coverage   91.17%   91.21%   +0.04%     
==========================================
  Files          61       61              
  Lines       11862    11898      +36     
==========================================
+ Hits        10815    10853      +38     
+ Misses       1047     1045       -2

Impacted Files	Coverage Δ
src/freadR.c	93.24% <ø> (ø)	⬆️
src/fread.c	95.69% <100%> (+0.14%)	⬆️
src/forder.c	94.21% <0%> (+0.26%)	⬆️

---

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 3a935d8...ef87fe5. Read the comment docs.