Currently, workspace is an attribute of IRB::Context in most use cases.
But when some workspace commands are used, like `pushws` or `popws`, a
workspace will be created and used along side with the original workspace
attribute.

This complexity is not necessary and will prevent us from expanding
multi-workspace support in the future.

So this commit introduces a @workspace_stack ivar to IRB::Context so IRB
can have a more natural way to manage workspaces.