Consider printing warning in non-JIT mode if any single invocation takes >100ms

[dtolnay]

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In the non-JIT runtime let's time how long each macro call takes and print a message if any one takes too long, pointing to the JIT installation instructions and explaining that it would likely improve your compile time.

We'll want the warning to somehow appear only once per compilation if possible. Also we'll want some way to silence it in case the user can't install JIT for some reason.