Generalise the JSON "Container" type

Currently, we have some types that we kept as dictionaries in the
objects representation (due to inconsistent behaviour, or other
technical reasons). This should be improved in the future, but in the
meantime we can improve the Container type by allowing its keys to be
non-strings; this is needed, for example, for better safe in
DiskParams type, where we don't want arbitrary strings as keys, but
only the actually defined types.

To implement this change, we generalise the type (Container →
GenericContainer), and in the process we introduce a type class for
"things that can be represented as strings". This is very similar to a
combination of Read and Show, but with custom string
representation. The new type class is needed because in JSON
representation, object keys must be strings, so we need to be able to
serialised/deserialise the generic keys to/from plain strings.

We also add the instance for DiskTemplate at the same time.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Adeodato Simo <dato@google.com>