Annotate Kernel#class as inline (#3250)

```
$ benchmark-driver -v --rbenv 'before;after;before --jit;after --jit' benchmark/mjit_class.yml --repeat-count=4
before: ruby 2.8.0dev (2020-06-23T07:09:54Z master 37a2e48d76) [x86_64-linux]
after: ruby 2.8.0dev (2020-06-23T17:29:56Z inline-class 0ff147c007) [x86_64-linux]
before --jit: ruby 2.8.0dev (2020-06-23T07:09:54Z master 37a2e48d76) +JIT [x86_64-linux]
after --jit: ruby 2.8.0dev (2020-06-23T17:29:56Z inline-class 0ff147c007) +JIT [x86_64-linux]
Calculating -------------------------------------
                         before       after  before --jit  after --jit
    mjit_class(self)    39.219M     40.060M       53.502M      69.202M i/s -     40.000M times in 1.019915s 0.998495s 0.747631s 0.578021s
       mjit_class(1)    39.567M     41.242M       52.100M      68.895M i/s -     40.000M times in 1.010935s 0.969885s 0.767749s 0.580591s

Comparison:
                 mjit_class(self)
         after --jit:  69201690.7 i/s
        before --jit:  53502336.4 i/s - 1.29x  slower
               after:  40060289.1 i/s - 1.73x  slower
              before:  39218939.2 i/s - 1.76x  slower

                    mjit_class(1)
         after --jit:  68895358.6 i/s
        before --jit:  52100353.0 i/s - 1.32x  slower
               after:  41241993.6 i/s - 1.67x  slower
              before:  39567314.0 i/s - 1.74x  slower
```

1 files changed, 21 insertions, 0 deletions

diff --git a/kernel.rb b/kernel.rb
index e7d955fe45..d00ba3a809 100644
--- a/kernel.rb
+++ b/kernel.rb
@@ -1,6 +1,27 @@
 module Kernel
   #
   #  call-seq:
+  #     obj.class    -> class
+  #
+  #  Returns the class of <i>obj</i>. This method must always be called
+  #  with an explicit receiver, as #class is also a reserved word in
+  #  Ruby.
+  #
+  #     1.class      #=> Integer
+  #     self.class   #=> Object
+  #--
+  # Equivalent to \c Object\#class in Ruby.
+  #
+  # Returns the class of \c obj, skipping singleton classes or module inclusions.
+  #++
+  #
+  def class
+    Primitive.attr! 'inline'
+    Primitive.cexpr! 'rb_obj_class(self)'
+  end
+
+  #
+  #  call-seq:
   #     obj.clone(freeze: nil) -> an_object
   #
   #  Produces a shallow copy of <i>obj</i>---the instance variables of