Feature request: Logging eval_tidy

[cfhammill]

As discussed in r-lib/rlang#678 I recently needed to log what symbols are getting hit in my data_mask using eval_tidy. @lionel- thought this might be a more appropriate home.

I have prototype using makeActiveBinding, although Lionel suggested there's a cleaner alternative using objectable.

The protoype is here: https://gist.github.com/cfhammill/4244e29bc667455a2406bf972e4287c6

I'm happy to open this as a PR with the active bindings version, although I'm not sure if/when I would be able to switch this to objectable.