wip(captp): add new hooks

[kumavis]

Changes to captp required to support ExtRefController in https://github.com/endojs/endo/pull/2473

Captp

• new hooks:
  • gcHook: called when remote says they dont need your export
  • onBeforeImportHook: allows you to specify a presence for a slot (similar to importHook)
  • onBeforeExportHook: allows you to specify a slot for a value (similar to exportHook)
  • missingExportHook: allows you to fill in an export missing from the table
• new methods:
  • importSlot: you provide a new presence for a slot and captp registers it
  • exportValue: you specify a value and slot and it is added to the export table
• newly exposed method:
  • makeRemoteKit: sets up the HandledPromie for communicating with a remote slot, does not register the presence in the table

[kumavis]

on the call it was suggested this could be done another way. maybe by all of:

• passing in import/export tables
  • replaces opts.gcHook (export table)
  • replaces opts.missingExportHook (export table)
  • replaces captp.exportValue (export table)
  • replaces captp.importSlot (import table)
• providing a slot provider
  • replaces opts.onBeforeExportHook (retrieving slot)
• providing a presence creator
  • replaces opts.onBeforeExportHook (creating a presence)
• still need captp.makeRemoteKit

[kumavis]

replaced by https://github.com/endojs/endo/pull/2564