Move definition of Passable type from notifier package to marshal.

[erights]

As of #1949 we have a temporary definition of the Passable type at

agoric-sdk/packages/notifier/src/types.js

Lines 11 to 17 in f83967e

	/**
	* @typedef {*} Passable
	* A value that can be passed between vats.
	* TODO This type should be exported by `@agoric/marshal` and imported here.
	* TODO For these tools to be used for distributed operation, we need a less
	* precise comparison than `t.is` when testing that these values have arrived.
	*/

Instead, the marshal package needs to be typed, and it should be the one to export a Passable type. Such a typed marshal package can and should be extracted from #1905 's https://github.com/Agoric/agoric-sdk/tree/amount-patterns/packages/marshal/src