JSPRC pipelining support for non-plain-objects

These seem like bugs.

At present we only support pipelining on bare objects, Functions, and RpcTargets. I guess we need to extend it to support Maps, Arrays, and other serializable types.

It sounds like there might be an additional bug with Arrays where numeric indexes aren't being picked up by the wildcard property. I would have expected them to be stringified per JavaScript tradition, but maybe that doesn't actually happen here...

_Originally posted by @kentonv in https://github.com/cloudflare/workerd/pull/1870#discussion_r1538072982_