made it possible to set index in strings

Rev: lib/modules/Yabu.pmod/test.pike:1.3
Rev: src/builtin_functions.c:1.111
Rev: src/docode.c:1.38
Rev: src/docode.h:1.7
Rev: src/interpret.c:1.85
Rev: src/main.c:1.53
Rev: src/module_support.c:1.12
Rev: src/object.c:1.52
Rev: src/operators.c:1.33
Rev: src/operators.h:1.5
Rev: src/stralloc.c:1.36
Rev: src/stralloc.h:1.16
Rev: src/testsuite.in:1.114