Function parse_quantifier 

fn parse_quantifier(cx: &mut ParseCtxt<'_>) -> ParseResult<Expr>

⟨quant⟩ := forall ⟨refine_param⟩ in ⟨int⟩..⟨int⟩ ⟨block⟩
         | exists ⟨refine_param⟩ in ⟨int⟩..⟨int⟩ ⟨block⟩
         | forall ⟨refine_param⟩                 ⟨block⟩
         | exists ⟨refine_param⟩                 ⟨block⟩