Prevent new signal connections after build/simulation has begun

## Motivation
Build-time uses signal connectivity to derive hierarchy, so new connections after build could cause strange mismatches between generated code and simulation behavior. Similarly, if signals are connected after the simulation has started, the behavior of the hardware could change. It's probably best to add some checks to prevent this type of things from happening.

## Desired solution
Prevent signal connectivity from changing after build and simulation has begun.

## Alternatives considered
Leave it as-is, with users responsible for any weird decisions they make.