vm_getivar() provides fastpath for T_OBJECT by caching an index
of ivar. This patch also provides fastpath for FL_EXIVAR objects.
FL_EXIVAR objects have an each ivar array and index can be cached
as T_OBJECT. To access this ivar array, generic_iv_tbl is exposed
by rb_ivar_generic_ivtbl() (declared in variable.h which is newly
introduced).

Benchmark script:

Benchmark.driver(repeat_count: 3){|x|
  x.executable name: 'clean', command: %w'../clean/miniruby'
  x.executable name: 'trunk', command: %w'./miniruby'

  objs = [Object.new, 'str', {a: 1, b: 2}, [1, 2]]

  objs.each.with_index{|obj, i|
    rep = obj.inspect
    rep = 'Object.new' if /\#/ =~ rep
    x.prelude str = %Q{
      v#{i} = #{rep}
      def v#{i}.foo
        @iv # ivar access method (attr_reader)
      end
      v#{i}.instance_variable_set(:@iv, :iv)
    }
    puts str
    x.report %Q{
      v#{i}.foo
    }
  }
}

Result:

      v0.foo # T_OBJECT

               clean:  85387141.8 i/s
               trunk:  85249373.6 i/s - 1.00x  slower

      v1.foo # T_STRING

               trunk:  57894407.5 i/s
               clean:  39957178.6 i/s - 1.45x  slower

      v2.foo # T_HASH

               trunk:  56629413.2 i/s
               clean:  39227088.9 i/s - 1.44x  slower

      v3.foo # T_ARRAY

               trunk:  55797530.2 i/s
               clean:  38263572.9 i/s - 1.46x  slower