Undo/redo (#659)

* immutable crossfilter

* first cut at reducer refactor with cascade model

* add initial redo/undo implementation

* small optimization

* integrate expression with history

* add tests for new reducers and fix a couple of small initialization bugs

* treat tiny lasso selections as a clear

* better function name for clarity

* fix undo for differential expression

* remove logging

* fix regression due to bad merge

* cleanup and comments for clarity

* improve undoable configuration for flexibility

* fix stale comments

* remove debugging code from production build

* rename categoricalSelectionState

* rename file

* improve comments

client/src/reducers/cascade.js

@@ -0,0 +1,44 @@
+export default function cascadeReducers(arg) {
+  /*
+  Combined a set of cascading reducers into a single reducer.  Cascading
+  reducers are reducers which may rely on state computed by another reducer.
+  Therefore, they:
+  - must be composed in a particular order (currently, this is a simple
+    linear list of reducers, run in list order)
+  - must have access to partially updated "next state" so they can further
+    derive state.
+
+  Parameter is one of:
+  - a Map object
+  - an array of tuples, [ [key1, reducer1], [key2, reducer2], ... ]
+  Ie,  cascadeReducers([ ["a", reduceA], ["b", reduceB] ])
+
+  Each reducer will be called with the sigature:
+      (prevState, action, sharedNextState, sharedPrevState) => newState
+
+  cascadeReducers will build a composite newState object, much
+  like combinedReducers.  Additional semantics:
+  - reducers guaranteed to be called in order
+  - each reducer will receive shared objects
+  */
+  const reducers = arg instanceof Map ? arg : new Map(arg);
+  const reducerKeys = [...reducers.keys()];
+  return (prevState, action) => {
+    const nextState = {};
+    let stateChange = false;
+    for (let i = 0, l = reducerKeys.length; i < l; i += 1) {
+      const key = reducerKeys[i];
+      const reducer = reducers.get(key);
+      const prevStateForKey = prevState ? prevState[key] : undefined;
+      const nextStateForKey = reducer(
+        prevStateForKey,
+        action,
+        nextState,
+        prevState
+      );
+      nextState[key] = nextStateForKey;
+      stateChange = stateChange || nextStateForKey !== prevStateForKey;
+    }
+    return stateChange ? nextState : prevState;
+  };
+}