reworked trampolines to cause less circular references, solves [bug 1937]

Rev: src/apply_low.h:1.4
Rev: src/docode.c:1.134
Rev: src/interpret_functions.h:1.95
Rev: src/language.yacc:1.262
Rev: src/las.c:1.269
Rev: src/las.h:1.53
Rev: src/pike_types.h:1.73
Rev: src/program.c:1.378