Use #in? instead of #includes? in several places

Because:

1. it’s more semantically descriptive
2. it’s shorter

src/ameba/ast/variabling/variable.cr

@@ -85,7 +85,7 @@ module Ameba::AST
       consumed_branches = Set(Branch).new
 
       assignments.reverse_each do |assignment|
-        next if consumed_branches.includes?(assignment.branch)
+        next if assignment.branch.in?(consumed_branches)
         assignment.referenced = true
 
         break unless branch = assignment.branch

src/ameba/rule/lint/duplicated_require.cr

@@ -23,7 +23,7 @@ module Ameba::Rule::Lint
     def test(source)
       nodes = AST::TopLevelNodesVisitor.new(source.ast).require_nodes
       nodes.each_with_object([] of String) do |node, processed_require_strings|
-        issue_for(node, MSG % node.string) if processed_require_strings.includes?(node.string)
+        issue_for(node, MSG % node.string) if node.string.in?(processed_require_strings)
         processed_require_strings << node.string
       end
     end

src/ameba/rule/style/variable_names.cr

@@ -55,7 +55,7 @@ module Ameba::Rule::Style
       private getter var_locations = [] of Crystal::Location
 
       def visit(node : Crystal::Var)
-        !var_locations.includes?(node.location) && super
+        !node.location.in?(var_locations) && super
       end
 
       def visit(node : Crystal::InstanceVar | Crystal::ClassVar)