right-side operator overloading implemented

Rev: src/array.c:1.18
Rev: src/interpret.c:1.49
Rev: src/lex.c:1.28
Rev: src/modules/Gmp/mpz_glue.c:1.23
Rev: src/modules/Gmp/testsuite.in:1.7
Rev: src/operators.c:1.20
Rev: src/program.c:1.43
Rev: src/program.h:1.21