Clarify semantics of fancy integer division functions

The functions modulo_floor, modulo_ceil, div_floor and div_ceil of the integer_functions exhibit unclear rounding behaviour. Their names and docstrings indicate mathematical rounding behaviour ("down" is negative infinity), while their implementation performs zero-oriented rounding ("down" is toward zero), as this is the default behaviour of C / and %.

We should clarify the semantics of these functions and adapt docstring, implementation, or both.

See also the discussion here: !368 (merged)