add (start) primitive beneath (run) and (succeeds?)

[vito]

• takes a thunk and a callback function
• runs the thunk concurrently
• calls the callback with a boolean indicating success or failure
• returns a function which can be called to wait for the thunk and return the result of the callback

so (succeeds? thunk) is implemented as ((start thunk id))

when the callback returns an error, the underlying error will be tacked on to the end; that way (run) can wrap the error with a higher-level message, while preserving the lower-level details