htools: improvements to JSON deserialisation

This fixes two problems:

- first, when we deserialise a big object, showing its value is not
  useful, as it will hide the actual error message
- second, we shouldn't deserialise a container at once, because then
  we will lose the detail of which 'key' failed to deserialise; we
  change to manual deserialisation of each key/value pair, so that we
  can keep this information

The last point requires that we import JSON.hs into THH.hs, in order
not to duplicate functionality.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>