Ticket #41: support StructField in add_field().

Fix various small bugs too.
Ticket #41: implement DelimitedSequence.

Given  {'x': [{'x': 1000}] * 10000}, PyPy, before:

    100 loops, best of 3: 7.94 msec per loop

PyPy, after:

    1000 loops, best of 3: 282 usec per loop

Cost of a delimited list is 3 allocations, could maybe improve on this.