Decouple tracing activation and creating a trace signal?

[ptaoussanis]

For ease-of-use of the common case, trace! and spy! currently do TWO things:

1. They activate tracing for the given run form, AND
2. They create a signal after the given run form completes

BOTH happen if signal IS allowed by filtering.
NEITHER happens if signal is NOT allowed by filtering.

This design is a reasonable place to start, but it'd be nice for advanced users to be able to decouple (1) and (2) for extra control.

In principle shouldn't be too difficult - would just need to extract the trace-activation code from signal! so that it can be called independently.

But would want to give this some proper thought.

Will investigate further if there's actually demand (👍's).