De-hardcode chemical metabolism of StomachComponent (#437)

* Move chemical reaction effects into Chemistry/ReactionEffects/ subfolder

* Replace hardcoded StomachComponent metabolism with IMetabolizable

The benefits of this approach are that reagent metabolism effects are not hardcoded into StomachComponent, and metabolism effects can be more easily chained together in yaml prototypes, and reagents can have different metabolism rates. One problem with this approach is that getting metabolism rates slower than 1u / second is impossible. Implementing #377 should resolve that problem.

* Fix DefaultFood and DefaultDrink so they remove reagent regardless of Hunger/ThirstComponent presence

Previously if neither of those were present the reagents wouldn't be removed from the stomach. This fixes that.

* Additional comment on function

* Make metabolizer interface implementations explicit

Also removed some unused using statements

* Make StomachComponent._reagentDeltas readonly

* Fix misleading variable names and docs for metabolizables

Changes one of the arguments for `IMetabolizable.Metabolize()` to be called `tickTime` instead of `frameTime` to more accurately reflect it's purpose. It's not really the frametime, but the time since the last metabolism tick. Also updated and expanded the docs to reflect this and to be more clear.

Content.Shared/Chemistry/DefaultMetabolizable.cs

@@ -0,0 +1,27 @@
+using System;
+using Content.Shared.Interfaces.Chemistry;
+using Robust.Shared.Interfaces.GameObjects;
+using Robust.Shared.Interfaces.Serialization;
+using Robust.Shared.Serialization;
+
+namespace Content.Shared.Chemistry
+{
+    //Default metabolism for reagents. Metabolizes the reagent with no effects
+    class DefaultMetabolizable : IMetabolizable
+    {
+        //Rate of metabolism in units / second
+        private int _metabolismRate = 1;
+        public int MetabolismRate => _metabolismRate;
+
+        void IExposeData.ExposeData(ObjectSerializer serializer)
+        {
+            serializer.DataField(ref _metabolismRate, "rate", 1);
+        }
+
+        int IMetabolizable.Metabolize(IEntity solutionEntity, string reagentId, float tickTime)
+        {
+            int metabolismAmount = (int)Math.Round(MetabolismRate * tickTime);
+            return metabolismAmount;
+        }
+    }
+}