Add a read-only job queue module

This patch adds implementation for a read-only job queue module,
together with "full" test (as full as can be in a lazy language…).

One note about the behaviour of the job queue is the handling of
opcodes that fail validation: the 'input' opcode actually is a
meta-type, which can hold either a real opcode or a plain JSValue, so
that we can still load jobs with invalid opcodes for querying. The
only downside of this is that, as opposed to Python code, we can't
show the correct summary for such an opcode - we try to parse the
OP_ID but not the extended OP_DSC_FIELD-equivalent.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>