Define circuit value semantics

[kpreid]

We need specific rules for circuit values and operations on them so that we don't end up dragging in the entirety of JavaScript by accident. Define:

• The set of permitted values.
• The definition of equality on those values (for count).
• The definition of truth on those values (for nor, gate, indicator).
• The method of combining multiple outputs feeding to one input (for junctions and multi-input blocks).

Don't forget to update the manual.