Implement some way to express that a signal doesn't need to be evaluated

[colinbdclark]

Currently, we have a "Value" signal that simply outputs a buffer containing a static value, though it can be updated arbitrarily by the user. However, there's no way to express that the value is constant throughout the lifetime of the program. As a result, we pay some cost in having to continually evaluate all signals at runtime.

This might be as simple as introducing a ConstantValue signal, but it would be nice for this to be a qualifier for any type of signal.

This may or may not dovetail with performance optimizations that could be implemented to only evaluate signals such as Value (which may only be updated rarely) when necessary.