Replace WatchExprBreakpoint with WatchIVarBreakpoint

[st0012]

Based on the discussion in #49, having WatchExprBreakpoint for watch all expression changes costs too much on performance but provide limited benefit to the users. But because watching instance variables for state changes is still valuable for debugging, we're going to support this particular case.

So in this PR, I made these changes:

• Replace WatchExprBreakpoint with WatchIVarBreakpoint.
• watch expr is no longer supported.
• watch @ivar is more accurate and cheaper.

---

About WatchIVarBreakpoint

Currently, when watching instance variables, the breakpoint isn't aware of the instance/class it's tracking on. So it could incorrectly track instance variable change (with the same name) in a different class.

For example, if you debug this script:

class Student
  attr_accessor :name

  def initialize(name)
    @name = name
    binding.bp(command: "watch @name ;; c") # should only watch Student's @name change
  end
end

class Teacher
  attr_accessor :name

  def initialize(name)
    @name = name
  end
end

s1 = Student.new("John")
t1 = Teacher.new("Jane") # this shouldn't trigger a stop
s1.name = "Josh"
"foo"

You'll find that it stops at Teacher's @name assignments as well, even though the script should only monitor Student's @name.

✦ ❯ exe/rdbg target.rb
[1, 10] in target.rb
=>    1| class Student
      2|   attr_accessor :name
      3|
      4|   def initialize(name)
      5|     @name = name
      6|     binding.bp(command: "watch @name ;; c")
      7|   end
      8| end
      9|
     10| class Teacher
=>#0    <main> at target.rb:1

(rdbg) c
[1, 10] in target.rb
      1| class Student
      2|   attr_accessor :name
      3|
      4|   def initialize(name)
      5|     @name = name
=>    6|     binding.bp(command: "watch @name ;; c")
      7|   end
      8| end
      9|
     10| class Teacher
=>#0    Student#initialize(name="John") at target.rb:6
  #1    [C] Class#new at target.rb:18
  #2    <main> at target.rb:18
(rdbg:init) watch @name
@name = John
#0 watch bp: @name = John
(rdbg:init) c
[14, 22] in target.rb
     14|     @name = name
     15|   end
     16| end
     17|
     18| s1 = Student.new("John")
=>   19| t1 = Teacher.new("Jane")
     20| s1.name = "Josh"
     21| "foo"
     22|
=>#0    <main> at target.rb:19

Stop by #0 watch bp: @name = John ->

(rdbg) c
[10, 19] in target.rb
     10| class Teacher
     11|   attr_accessor :name
     12|
     13|   def initialize(name)
     14|     @name = name
=>   15|   end
     16| end
     17|
     18| s1 = Student.new("John")
     19| t1 = Teacher.new("Jane")
=>#0    Teacher#initialize(name="Jane") at target.rb:15 #=> "Jane"
  #1    [C] Class#new at target.rb:19
  #2    <main> at target.rb:19

Stop by #0 watch bp: @name =  -> Jane

(rdbg) c
[15, 22] in target.rb
     15|   end
     16| end
     17|
     18| s1 = Student.new("John")
     19| t1 = Teacher.new("Jane")
=>   20| s1.name = "Josh"
     21| "foo"
     22|
=>#0    <main> at target.rb:20

Stop by #0 watch bp: @name = Jane ->

(rdbg) c

After

✦ ❯ exe/rdbg target.rb
[1, 10] in target.rb
=>    1| class Student
      2|   attr_accessor :name
      3|
      4|   def initialize(name)
      5|     @name = name
      6|     binding.bp(command: "watch @name ;; c")
      7|   end
      8| end
      9|
     10| class Teacher
=>#0    <main> at target.rb:1

(rdbg) c
[1, 10] in target.rb
      1| class Student
      2|   attr_accessor :name
      3|
      4|   def initialize(name)
      5|     @name = name
=>    6|     binding.bp(command: "watch @name ;; c")
      7|   end
      8| end
      9|
     10| class Teacher
=>#0    Student#initialize(name="John") at target.rb:6
  #1    [C] Class#new at target.rb:18
  #2    <main> at target.rb:18
(rdbg:init) watch @name
#<Student:0x00007fc3c68cc8c8> @name = John
#0 watch bp: #<Student:0x00007fc3c68cc8c8> @name = John
(rdbg:init) c
[16, 22] in target.rb
     16| end
     17|
     18| s1 = Student.new("John")
     19| t1 = Teacher.new("Jane")
     20| s1.name = "Josh"
=>   21| "foo"
     22|
=>#0    <main> at target.rb:21

Stop by #0 watch bp: #<Student:0x00007fc3c68cc8c8> @name = John -> Josh

(rdbg) c

Another benefit of the change is that we can show what instance's ivar is changed:

#<Student:0x00007fc3c68cc8c8> @name = John

(I plan to work on styling it in another PR)

[ko1]

This proposal is strongly connected with "how to design watch command". #49

So wait for merging.

[st0012]

@ko1 btw this breakpoint is immune from the issue I mentioned in #61 (comment)