Add time constraints

[istathar]

Introduce the within control structure keyword to specify a subscope that must be completed within the constraint of a resource budget. Appears as follows:

    4.  Roast turkey
        { within 2 hours }
            a.  Warm oven
            b.  Put bird in to cook

Add a new syntax for "cost literals", $( ... ) for creating a new Intratempse value. This can be written in descriptive prose (at the end of a step) to indicate the resource "cost" of a step, be it in time, currency, or some other amount.

            c.  Roast dinner $(3 hours)

This syntax is also available as a literal value constructor in an expression.

The argument of within is implicitly coerced to an Intratempse if a bare Quanticle literal is written there.