Also make `jupyter-wait-until-startup` private by renaming to
`jupyter--wait-until-startup`. Startup messages are only sent once at kernel
startup, since the only way to start a kernel is through a
`jupyter-kernel-manager` it makes more sense to only allow a kernel manager to
wait for a startup message.
This way when creating kernel clients with `jupyter-kernel-manager`, we can set
the `parent-instance` of the newly created client to the manager thereby
inheriting the `jupyter-session` from the kernel manager.
Also have `jupyter-channel` objects inherit from a `jupyter-connection` so that
we can define `jupyter-send` and `jupyter-recv` methods for channels in the future.
- `jupyter-client.el` only contains client related code
- `jupyter-kernel-manager` related code is placed in `jupyter-kernel-manager.el`
- Socket creating functions and generating connection info plist function are
placed in `jupyter-connection.el`. This also contains the
`jupyter-connection` class.
- Kernelspec related functions are placed in `jupyter-kernelspec.el`
- Move general utility functions and variables requires necessary for `jupyter`
into `jupyter-base.el`. This also contains the `jupyter-session` and
`jupyter-request` struct definitions.
