Implement lambda-based Scenario variables

Scenario's 'set' scenelet now takes a function as its argument: Scenario
calls the function at scene time to get the value.  This enables using
Sphere APIs like GetPersonX() to set a variable.  Also made Scenario
call the input handler for a scenelet before calling its updater,
instead of after.
Scenario: Implement lambda conditionals

This enables using normally-declared JS variables and functions as part
of a conditional without relying on clunky scene variables.  Also
removed the redundant doUntil scenelet.