Use tuple instead of an array

spec/spec_helper.cr

@@ -241,7 +241,7 @@ module Ameba
   end
 
   class TestNodeVisitor < Crystal::Visitor
-    NODES = [
+    NODES = {
       Crystal::NilLiteral,
       Crystal::Var,
       Crystal::Assign,
@@ -255,7 +255,7 @@ module Ameba
       Crystal::MacroLiteral,
       Crystal::Expressions,
       Crystal::ControlExpression,
-    ]
+    }
 
     def initialize(node)
       node.accept self

src/ameba/ast/visitors/node_visitor.cr

@@ -2,7 +2,7 @@ require "./base_visitor"
 
 module Ameba::AST
   # List of nodes to be visited by Ameba's rules.
-  NODES = [
+  NODES = {
     Alias,
     IsA,
     Assign,
@@ -27,7 +27,7 @@ module Ameba::AST
     When,
     While,
     Until,
-  ]
+  }
 
   # An AST Visitor that traverses the source and allows all nodes
   # to be inspected by rules.

src/ameba/ast/visitors/scope_visitor.cr

@@ -4,7 +4,7 @@ module Ameba::AST
   # AST Visitor that traverses the source and constructs scopes.
   class ScopeVisitor < BaseVisitor
     # Non-exhaustive list of nodes to be visited by Ameba's rules.
-    NODES = [
+    NODES = {
       ClassDef,
       ModuleDef,
       EnumDef,
@@ -17,7 +17,7 @@ module Ameba::AST
       Block,
       Macro,
       MacroFor,
-    ]
+    }
 
     SUPER_NODE_NAME  = "super"
     RECORD_NODE_NAME = "record"