use a deque to implement a stack

CHANGELOG.md

@@ -1,3 +1,9 @@
+# 0.2.0
+
+- **(breaking-change)** State refactor ([#15](https://github.com/RomainFranceschini/quartz/pull/15))
+  - `state_var : Type = value` replaced by `state { var : Type = value }`
+  - State variables are no longer part of the model, the model now has a reference to its companion class `State`.
+
 # 0.1.0
 
 - First version

src/quartz/processor_allocator.cr

@@ -5,7 +5,7 @@ module Quartz
     class ProcessorAllocationError < Exception; end
 
     @simulation : Simulation
-    @stack : Array(Coordinator)
+    @stack : Deque(Coordinator)
     @root_model : CoupledModel?
 
     @root_coordinator : Coordinator?
@@ -15,11 +15,11 @@ module Quartz
     end
 
     def initialize(@simulation, @root_model)
-      @stack = Array(Coordinator).new
+      @stack = Deque(Coordinator).new
     end
 
     def initialize(@simulation, parent_processor : Coordinator)
-      @stack = Array(Coordinator).new
+      @stack = Deque(Coordinator).new
       @stack.push parent_processor
     end