Changed the definition of Pike_fp->context to a pointer.

This should reduce the overhead for apply() somewhat.

Rev: src/apply_low.h:1.33
Rev: src/builtin.cmod:1.194
Rev: src/interpret.c:1.386
Rev: src/interpret.h:1.171
Rev: src/interpret_functions.h:1.200
Rev: src/modules/Gmp/mpz_glue.c:1.171
Rev: src/object.c:1.282
Rev: src/object.h:1.93
Rev: src/opcodes.c:1.170
Rev: src/operators.c:1.227
Rev: src/program.c:1.653