Use stack_pop_keep_top and stack_pop_2_elems_keep_top where possible.

Rev: src/builtin_functions.c:1.488
Rev: src/interpret_functions.h:1.150
Rev: src/iterators.cmod:1.40
Rev: src/modules/files/stat.c:1.28
Rev: src/opcodes.c:1.144
Rev: src/operators.c:1.176