RubyVM.keep_script_lines

`RubyVM.keep_script_lines` enables to keep script lines
for each ISeq and AST. This feature is for debugger/REPL
support.

```ruby
RubyVM.keep_script_lines = true
RubyVM::keep_script_lines = true

eval("def foo = nil\ndef bar = nil")
pp RubyVM::InstructionSequence.of(method(:foo)).script_lines
```

test/ruby/test_rubyvm.rb

@@ -15,4 +15,56 @@ class TestRubyVM < Test::Unit::TestCase
     assert_raise(ArgumentError){ RubyVM.stat(:unknown) }
     assert_raise_with_message(ArgumentError, /\u{30eb 30d3 30fc}/) {RubyVM.stat(:"\u{30eb 30d3 30fc}")}
   end
+
+  def parse_and_compile
+    script = <<~RUBY
+      a = 1
+      def foo
+        b = 2
+      end
+      1.times{
+        c = 3
+      }
+    RUBY
+
+    ast = RubyVM::AbstractSyntaxTree.parse(script)
+    iseq = RubyVM::InstructionSequence.compile(script)
+
+    [ast, iseq]
+  end
+
+  def test_keep_script_lines
+    prev_conf = RubyVM.keep_script_lines
+
+    # keep
+    RubyVM.keep_script_lines = true
+
+    ast, iseq = *parse_and_compile
+
+    lines = ast.script_lines
+    assert_equal Array, lines.class
+
+    lines = iseq.script_lines
+    assert_equal Array, lines.class
+    iseq.each_child{|child|
+      assert_equal lines, child.script_lines
+    }
+
+    # don't keep
+    RubyVM.keep_script_lines = false
+
+    ast, iseq = *parse_and_compile
+
+    lines = ast.script_lines
+    assert_equal nil, lines
+
+    lines = iseq.script_lines
+    assert_equal nil, lines
+    iseq.each_child{|child|
+      assert_equal lines, child.script_lines
+    }
+
+  ensure
+    RubyVM.keep_script_lines = prev_conf
+  end
 end