print() and print_when() could be more ergonomic to use

[steven-johnson]

These are great and essential tools for debugging, but they could be a lot better:

• They are painful to conditionally insert into the code because the syntax is very intrusive
• Turning them on and off requires recompiles

Thought: what if we added a scheduling directive for print_when() that would inject the print when the func is evaluated? (No need for print() since that's just print_when(true))? And added some optional controls via env var (eg print_when_env("ENV_VAR_NAME")?)

[abadams]

So an unconditional print is just Func::trace_stores. We could add an optional boolean Expr arg to it?

[steven-johnson]

So an unconditional print is just Func::trace_stores. We could add an optional boolean Expr arg to it?

Yes and know; when I use these for debugging I usually also want to print other data too