div and mod: Built-in functions

TODO: Document divmod properly, in particular the adherence to C++11 rules:

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A footnote to 5.6/4 said:

[C++03 footnote 74]: According to work underway toward the revision of ISO C, the preferred algorithm for integer division follows the rules defined in the ISO Fortran standard, ISO/IEC 1539:1991, in which the quotient is always rounded toward zero. In C++11 this behaviour is explicitly required rather than being “preferred”; the change is listed in the compatibility section:

[C++11: C.2.2]: Change: Specify rounding for results of integer / and % Rationale: Increase portability, C99 compatibility. Effect on original feature: Valid C++ 2003 code that uses integer division rounds the result toward 0 or toward negative infinity, whereas this International Standard always rounds the result toward 0.