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.

Hoogle v5 uses a different json representation for the data, requires a
new URL and also contains HTML markup inside the json fields.

This commit removes all markup to be able to handle the hoogle responses
using the same structure as with the previous response structure (from
hoogle v4).

GHC 8.4 and later have the `-Wpartial-fields` warning flag, so add it and
fix the warnings.

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.