Introduce cd command

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

Help message:

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

IRB uses a stack of workspaces to keep track of context(s), with `pushws` and `popws` commands to manipulate the stack.
The `cd` command is an attempt to simplify the operation and will be subject to change.

When given:
- an object, cd will use that object as the new context by pushing it onto the workspace stack.
- "..", cd will leave the current context by popping the top workspace off the stack.
- no arguments, cd will move to the top workspace on the stack by popping off all workspaces.

Examples:

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

lib/irb/command/cd.rb

@@ -0,0 +1,51 @@
+# 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]|..)
+
+        IRB uses a stack of workspaces to keep track of context(s), with `pushws` and `popws` commands to manipulate the stack.
+        The `cd` command is an attempt to simplify the operation and will be subject to change.
+
+        When given:
+        - an object, cd will use that object as the new context by pushing it onto the workspace stack.
+        - "..", cd will leave the current context by popping the top workspace off the stack.
+        - no arguments, cd will move to the top workspace on the stack by popping off all workspaces.
+
+        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