Provide better messages for code involving infix operators

[rossellhayes]

• When only one of solution or submitted is an infix, provide the whole context,
  • e.g. "I expected you to call mean() where you called (x + y)/2."
• When both solution and submitted are infixes, provide jus the operator,
  • e.g. "I expected you to call + where you called *."

Closes #84.

TODO

• [x] Consider #225

  • Closed by #327

• [ ] Consider #272

• [ ] Fix handling of chained infixes

  user <- "sqrt(1 + 2)"
  solution <- "sqrt(1 + 2 + 3)"
  code_feedback(user, solution)
  #> In `1 + 2`, I expected `1 + 2` where you wrote `1`.

• [ ] Don't reveal the contents of functions that are infix arguments

  user <- "b(1)"
  solution <- "b(1) + 2"
  code_feedback(user, solution)
  #> I expected you to call `b(1) + 2` where you called `b()`.

• [ ] Investigate how we give feedback on the base pipe (|>)

• [ ] Improve handling of repeated operators (e.g. !!, !!! and {{ }})

^{Created on 2023-02-16 with reprex v2.0.2}

[CLAassistant]

All committers have signed the CLA.