Introduce cd command

It's essentially a combination of pushws and popws commands that are
easier to use.

Help message:

```
Usage: cd ([target]|..)

When given an object, cd will move into that object, making it the current context.
When given "..", cd will leave the current context, moving back to the previous context.
When given no arguments, cd will move back to the top-level (main object) context.

Examples:

  cd Foo
  cd Foo.new
  cd @ivar
  cd ..
  cd
```

lib/irb/command/cd.rb

@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+module IRB
+  module Command
+    class CD < Base
+      category "Workspace"
+      description "Move into the given object or leave the current context."
+
+      help_message(<<~HELP)
+        Usage: cd ([target]|..)
+
+        When given an object, cd will move into that object, making it the current context.
+        When given "..", cd will leave the current context, moving back to the previous context.
+        When given no arguments, cd will move back to the top-level (main object) context.
+
+        Examples:
+
+          cd Foo
+          cd Foo.new
+          cd @ivar
+          cd ..
+          cd
+      HELP
+
+      def execute(arg)
+        case arg
+        when ".."
+          irb_context.pop_workspace
+        when ""
+          # TODO: decide what workspace commands should be kept, and underlying APIs should look like,
+          # and perhaps add a new API to clear the workspace stack.
+          prev_workspace = irb_context.pop_workspace
+          while prev_workspace
+            prev_workspace = irb_context.pop_workspace
+          end
+        else
+          begin
+            obj = eval(arg, irb_context.workspace.binding)
+            irb_context.push_workspace(obj)
+          rescue StandardError => e
+            warn "Error: #{e}"
+          end
+        end
+      end
+    end
+  end
+end

lib/irb/default_commands.rb

@@ -5,6 +5,7 @@
 require_relative "command/backtrace"
 require_relative "command/break"
 require_relative "command/catch"
+require_relative "command/cd"
 require_relative "command/chws"
 require_relative "command/context"
 require_relative "command/continue"
@@ -240,6 +241,8 @@ def load_command(command)
     _register_with_aliases(:irb_disable_irb, Command::DisableIrb,
       [:disable_irb, NO_OVERRIDE]
     )
+
+    register(:cd, Command::CD)
   end
 
   ExtendCommand = Command

test/irb/command/test_cd.rb

@@ -0,0 +1,65 @@
+require "tempfile"
+require_relative "../helper"
+
+module TestIRB
+  class CDTest < IntegrationTestCase
+    def setup
+      super
+
+      write_ruby <<~'RUBY'
+        class Foo
+          class Bar
+            def bar
+              "this is bar"
+            end
+          end
+
+          def foo
+            "this is foo"
+          end
+        end
+
+        binding.irb
+      RUBY
+    end
+
+    def test_cd
+      out = run_ruby_file do
+        type "cd Foo"
+        type "ls"
+        type "cd Bar"
+        type "ls"
+        type "cd .."
+        type "exit"
+      end
+
+      assert_match(/irb\(Foo\):002>/, out)
+      assert_match(/Foo#methods: foo/, out)
+      assert_match(/irb\(Foo::Bar\):004>/, out)
+      assert_match(/Bar#methods: bar/, out)
+      assert_match(/irb\(Foo\):006>/, out)
+    end
+
+    def test_cd_moves_top_level_with_no_args
+      out = run_ruby_file do
+        type "cd Foo"
+        type "cd Bar"
+        type "cd"
+        type "exit"
+      end
+
+      assert_match(/irb\(Foo::Bar\):003>/, out)
+      assert_match(/irb\(main\):004>/, out)
+    end
+
+    def test_cd_with_error
+      out = run_ruby_file do
+        type "cd Baz"
+        type "exit"
+      end
+
+      assert_match(/Error: uninitialized constant Baz/, out)
+      assert_match(/irb\(main\):002>/, out) # the context should not change
+    end
+  end
+end