(https://github.com/ruby/irb/pull/971)
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
```
4a0e0e89b7
