Add: Fortran hex_string subroutine

[acxz]

Replaces an inefficient internal write statement that was skewing the parse_int benchmark results.

Squash of #41 to run on benchmark ci

Closes #41

[acxz]

What the CI shows:

Before PR: See: https://github.com/JuliaLang/Microbenchmarks/runs/5503827600?check_suite_focus=true#step:17:23

c,parse_integers,0.16304
fortran,parse_integers,0.629203
julia,parse_integers,0.123102
rust,parse_integers,0.155706

After PR: See: https://github.com/JuliaLang/Microbenchmarks/runs/5513476775?check_suite_focus=true#step:17:23

c,parse_integers,0.16561
fortran,parse_integers,0.109366
julia,parse_integers,0.1196
rust,parse_integers,0.147733