enumerator_block_call was not safe for compaction because the Array
backing the argv was not pinned, so it could get moved during compaction
which would make argv point to somewhere else.

The test crashes when RGENGC_CHECK_MODE is turned on:

    TestEnumerator#test_with_index_under_gc_compact_stress
    check_rvalue_consistency: 0x55db0b399450 is not a Ruby object.
    test/ruby/test_enumerator.rb:133: [BUG] check_rvalue_consistency_force: there is 1 errors.
    ruby 3.3.0dev (2023-12-23T23:00:27Z master 50bf437341) [x86_64-linux]
    -- Control frame information -----------------------------------------------
    c:0034 p:---- s:0192 e:000187 CFUNC  :with_index
    c:0033 p:---- s:0185 e:000184 CFUNC  :each
    c:0032 p:---- s:0182 e:000181 CFUNC  :to_a
    c:0031 p:0055 s:0178 e:000175 BLOCK  test/ruby/test_enumerator.rb:133
    c:0030 p:0024 s:0172 e:000171 METHOD tool/lib/envutil.rb:242
    c:0029 p:0024 s:0167 e:000166 METHOD tool/lib/envutil.rb:251
    c:0028 p:0005 s:0160 e:000159 METHOD test/ruby/test_enumerator.rb:131
    ...
    -- C level backtrace information -------------------------------------------
    build/ruby(rb_print_backtrace+0x14) [0x55db0b1deb21] vm_dump.c:820
    build/ruby(rb_vm_bugreport) vm_dump.c:1151
    build/ruby(bug_report_end+0x0) [0x55db0b3a53a6] error.c:1042
    build/ruby(rb_bug_without_die) error.c:1042
    build/ruby(die+0x0) [0x55db0afc77c2] error.c:1050
    build/ruby(rb_bug) error.c:1052
    build/ruby(gc_move+0x0) [0x55db0afbada0] gc.c:1714
    build/ruby(check_rvalue_consistency+0xa) [0x55db0afef0c3] gc.c:1729
    build/ruby(is_markable_object) gc.c:4769
    build/ruby(gc_mark_stack_values) gc.c:6595
    build/ruby(rb_gc_mark_vm_stack_values) gc.c:6605
    build/ruby(rb_execution_context_mark+0x39) [0x55db0b1d8589] vm.c:3309
    build/ruby(thread_mark+0x15) [0x55db0b1a9805] vm.c:3381
    build/ruby(gc_mark_stacked_objects+0x6d) [0x55db0aff2c3d] gc.c:7564
    build/ruby(gc_mark_stacked_objects_all) gc.c:7602
    build/ruby(gc_marks_rest) gc.c:8797
    build/ruby(gc_marks+0xd) [0x55db0aff43d5] gc.c:8855
    build/ruby(gc_start) gc.c:9608
    build/ruby(rb_multi_ractor_p+0x0) [0x55db0aff5463] gc.c:9489
    build/ruby(rb_vm_lock_leave) vm_sync.h:92
    build/ruby(garbage_collect) gc.c:9491
    build/ruby(newobj_slowpath+0xcb) [0x55db0aff57ab] gc.c:2871
    build/ruby(newobj_slowpath_wb_protected) gc.c:2895
    build/ruby(newobj_of0+0x24) [0x55db0aff59e4] gc.c:2937
    build/ruby(newobj_of) gc.c:2947
    build/ruby(rb_wb_protected_newobj_of) gc.c:2962
    build/ruby(ary_alloc_embed+0x10) [0x55db0b2f3e40] array.c:668
    build/ruby(ary_new) array.c:709
    build/ruby(rb_ary_tmp_new_from_values) array.c:759
    build/ruby(rb_ary_new_from_values) array.c:771
    build/ruby(args_copy+0x18) [0x55db0b1bbb88] vm_args.c:158

Instead of `RUBY_REFERENCES_START` and `RUBY_REFERENCES_END`, so that
auto-indent works well.

Also move `struct` so that `typedef`-ed names can be used.