htools: add new template haskell system

This system based on explicit types instead of ad-hoc rules
(e.g. instead of deducing from "Maybe Int" an optional field, we now
can say explicitly OptionalField ''Int). In the first phase, this will
be used for the equivalent of lib/objects.py, which has slightly
different rules than luxi/opcodes.

We should look at merging the two systems later.

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