Limit the required interface for PMCs

[coke]

In the pdd03 design document there is the todo item (with context):

=item 5 C<FLAT> (P only)

If this bit is set on a PMC value, then the PMC must be an aggregate. The
contents of the aggregate, rather than the aggregate itself, will be passed.

If the C<NAMED> bit is also set, the aggregate will be used as a hash; its
contents, as key/value pairs, will be passed as named parameters. The PMC
must implement the full hash interface. {{ TODO: Limit the required
interface. }}

What is the required interface? How should it be limited? This, I
suspect, needs to be discussed and decided upon, and then documented
in the PDD.

Originally reported in  http://rt.perl.org/rt3/Ticket/Display.html?id=45367