phi syntax sugar is not tested

[yegor256]

Currently, we don't know whether syntax sugar works as expected in PhiSyntax. How about we create an additional parametrized test, which will use YAML files with two fields in each one:

sweet:
  {⟦ k(t) ↦ ⟦⟧ ⟧}
salty:
  {⟦ k ↦ ⟦ t ↦ ∅ ⟧ ⟧}

The test will parse the salty part and then print it back in sugared form. Also, the other way around.