Add more operators to list of infixes

[gadenbuie]

Adds more operators to our list of infixes, increasing coverage to include everything listed in ?base::Syntax.

Fixes #326

pkgload::load_all()

ex <- mock_this_exercise(
  .user_code = "TRUE | FALSE",
  .solution_code = "FALSE & FALSE"
) 

grade_this({
  fail_if_code_feedback()
  pass()
})(ex)
#> <gradethis_graded: [Incorrect]
#>   I expected you to call `&` where you called `|`.
#> >

[rossellhayes]

Looks good to me!

[rossellhayes]

Oh, before merging let's also add :=. It's not listed in ?Syntax but it is parsed as an operator.
Added support for := in bdf8fac.