More visitor stuff

[danfuzz]

This PR fleshes out BaseValueVisitor more completely, including:

• Giving it three different ways to be called:
  • synchronously, which will appropriately fail if any part of the visit turns out to be coded asynchronously.
  • asynchronously, with a visit-result promise chained to the result of the call.
  • asynchronously, with all visit results — including promises — wrapped, so that an asynchronous visit can be unambiguously differentiated from a visit that results in a promise. (Yay JavaScript!)
• Adding a bunch of tests.
• Fixing the original code which (of course) failed a number of the tests.