better type checking implemented (and a bug in the type checking fixed)

Rev: src/builtin_functions.c:1.76
Rev: src/docode.c:1.33
Rev: src/las.c:1.55
Rev: src/operators.c:1.28
Rev: src/pike_types.c:1.34
Rev: src/program.c:1.65
Rev: src/stralloc.c:1.28
Rev: src/svalue.h:1.13