fix: split marshal from its encoder

[erights]

NOTE: LOW PRIORITY. NOTHING IS BLOCKED ON THIS.

The encodePassable.js module in agoric-sdk has two interesting differences from the current marshal logic.

• It is a binary rather than JSON encoding, we may or may not have performance advantages. (We won't actually know until we measure.)
• It is parameterized at a lower level that marshal, providing more flexibility in the reuse of encoding logic.

If it is such a great lower level of abstraction for encapsulating most encoding concerns, could we use this abstraction boundary to split marshal up into separately understandable pieces, separating concerns? This PR does approximately that, with the difference that the original encodePassable encodes directly to string, where the PR's encodeToJSON encodes to a raw JSON tree, leaving it to the caller to stringify it. Leaving aside that difference, splitting marshal in this way would make it easier to experiment with a marshal that use the encodePassable binary encoding.

This separation would make it easier to address issues like
* Agoric/agoric-sdk#4334
* Agoric/agoric-sdk#4310
* Agoric/agoric-sdk#4311
* #997
* Agoric/agoric-sdk#2780

and is a prerequisite for #1260 , which is staged on this one.

[gibson042]

I like this separation! My biggest concerns are around maintaining the distinction between "JSON" (string) and "JSON-representable" (data structure) and naming/commenting accordingly (related: #1248). And we also need to resolve the issue around iface not being guaranteed to exist for every encoded remotable.

[erights]

@gibson042 thanks for the great suggestions! I think I dealt with all of them. PTAL.

[gibson042]

LGTM! Remaining comments are all minor, and mostly fixing typos.