remove module from `main`, and promote from `other-modules` to
`exported-modules` in `library`.

Fixes #971.

The IPython specification for the metadata does not
define that all fields in metadata must be textual.
Latest Jupyters lab uses that feature so the parsing
mechanism became incomplatible with the protocol.
We change Metadata to be an Aeson.Object and introduce
a 'Metadata' newtype in order to keep that explicit
and open for the later changes if needed.

Fixes #912.

Fixing these warnings required changes to the file names of the `MessageHeader`
struct which makes this commit an API change. The version in the cabal file
has been bumped accordingly.

Add `-Wunused-do-bind -Wunused-local-binds -Wunused-top-binds -Wunused-matches`
and fix all the things.

Add `-Wmissing-signatures` to `ghc-options`.

Add `-Wincomplete-patterns` to the `ghc-options` field of the cabal file
and fix all warnings.

This stops nbconvert from complaining.

It's a bit closer to a dictionary.

These messages are used by console frontends to check whether the
current input is complete or not. If it is not complete, the next line
is treated as the continuation of the code, and a '...' prompt is used
to show that.

To provide support for qtconsole, I have made it such that IHaskell
always responds to an is_complete_request by saying that the code is
complete, which only allows for single line inputs.