Added compiler environment class.

src/compiler/env.js

+
+// ALL Environments share the same program counter!
+var PC = 0;
+
+
+class Environment{
+  constructor(parent){
+    this.__parent = (parent instanceof Environment) ? parent : null;
+    this.__vars = {};
+  }
+
+  get PC(){return PC;}
+  set PC(val){
+    if (val < 0)
+      throw new RangeError("Program Counter value is out of bounds.");
+    PC = val;
+  } 
+
+  hasVar(name){
+    if (name in Object.keys(this.__vars))
+      return true;
+    if (this.__parent !== null)
+      return this.__parent.hasVar(name);
+    return false;
+  }
+
+  getVarValue(name){
+    if (name in Object.keys(this.__vars))
+      return this.__vars[name];
+    if (this.__parent !== null)
+      return this.__parent.getVarValue(name);
+    throw new Error("No var '" + name + "' defined.");
+  }
+
+  setVarValue(name, val){
+    if (!(name in Object.keys(this.__vars))){
+      if (this.__parent.hasVar(name))
+        return this.__parent.setVarVal(name, val);
+    }
+    this.__vars[name] = val;
+    return this;
+  }
+}
+
+
+module.exports = Environment;