Fixup warnings

[Riolku]

This fixes up three warnings:

• Wrong format specifier in the embedded shell.
• Using abs instead of std::abs in our arithmetic functions.
• Exporting types in our c_api.

Exporting structs only makes sense in C++, and only when there are methods to export (or typeinfo). Neither of those is true here.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (ede12a7) 89.63% compared to head (0b8cb7a) 89.63%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2138   +/-   ##
=======================================
  Coverage   89.63%   89.63%           
=======================================
  Files         988      988           
  Lines       35729    35729           
=======================================
+ Hits        32024    32026    +2     
+ Misses       3705     3703    -2     

Files	Coverage Δ
...include/function/arithmetic/arithmetic_functions.h	93.10% <100.00%> (ø)

... and 1 file with indirect coverage changes

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