`struct rb_calling_info::cd` is introduced and `rb_calling_info::ci` is replaced with it to manipulate the inline cache of iseq while method invocation process. So that `ci` can be acessed with `calling->cd->ci`. It adds one indirection but it can be justified by the following points: 1) `vm_search_method_fastpath()` doesn't need `ci` and also `vm_call_iseq_setup_normal()` doesn't need `ci`. It means reducing `cd->ci` access in `vm_sendish()` can make it faster. 2) most of method types need to access `ci` once in theory so that 1 additional indirection doesn't matter.