Support pretty printing

[deemp]

Add the --pretty option that makes normalizer pretty-print PHI programs.

Here's how EO prints programs.

{
  ⟦
    org ↦ ⟦
      eolang ↦ ⟦
        as-phi ↦ ⟦
          λ ⤍ Lorg_eolang_as_phi,
          x ↦ ∅
        ⟧,
        λ ⤍ Package
      ⟧,
      λ ⤍ Package
    ⟧
  ⟧
}