Add option to globally disable snoop precompilation

[baggepinnen]

This implements one of the suggestions from #298 and allows the user to opt out of all precompilation by setting the environment variable ENV["SNOOP_PRECOMPILE"] = false

[timholy]

The issue I see with this is that it may leave people in a complicated mix of *.ji files: those that happened to be written while SNOOP_PRECOMPILE=0 and those written when it defaulted to being on. That means TTFX could vary wildly depending on the specifics of your precompilation history. Morever, this doesn't seem to have a mechanism for ensuring that the precompiled versions get generated & used when you stop setting SNOOP_PRECOMPILE=0. The only option for a "clean house" would be to delete all the files in your .julia/compiled/v1.x directory and then precompile everything.

However, I think if we use Preferences.jl, all these negatives go away. Care to try an implementation using Preferences and see if it solves your problem?

[timholy]

Just checking in, is this something you're planning to do or should I marked this "help wanted"?

[baggepinnen]

I'd be happy to do this, I've been out-of-office for a while but will try to get to it after the backlog has been worked through.

[timholy]

With native-code compilation about to be merged, this will become quite urgent. I'll tackle it myself within a week unless you tell me otherwise.

[timholy]

I think we also want the ability to turn it off for specific packages, again under Preferences.jl control. Typically I think you'd want to turn it off for just the package you're actively working on.

[t-bltg]

Superseded by #334.