Remove QueryKeyMaker abstraction. (#4245)

It's nice to be able to assume that (sub)queries with equivalent structure
share the same object identities, but the cache will work without that
assumption, and enforcing that discipline is not free, both in terms of
runtime cost and in terms of bundle size.

In the future, the `graphql-tag` tag package should provide a version of
the `gql` template tag function that returns immutable structures that
share object identity where possible.

This commit alone saves 450ish bytes after minification and gzip!

packages/apollo-cache-inmemory/src/readFromStore.ts

@@ -43,7 +43,6 @@ import { wrap, CacheKeyNode } from './optimism';
 export { OptimisticWrapperFunction } from './optimism';
 
 import { DepTrackingCache } from './depTrackingCache';
-import { QueryKeyMaker } from './queryKeyMaker';
 
 export type VariableMap = { [name: string]: any };
 
@@ -94,8 +93,6 @@ type ExecSelectionSetOptions = {
 };
 
 export class StoreReader {
-  private keyMaker: QueryKeyMaker;
-
   constructor(
     private cacheKeyRoot = new CacheKeyNode,
   ) {
@@ -105,8 +102,6 @@ export class StoreReader {
       executeSelectionSet,
     } = reader;
 
-    reader.keyMaker = new QueryKeyMaker(cacheKeyRoot);
-
     this.executeStoreQuery = wrap((options: ExecStoreQueryOptions) => {
       return executeStoreQuery.call(this, options);
     }, {
@@ -122,7 +117,7 @@ export class StoreReader {
         // the cache when relevant data have changed.
         if (contextValue.store instanceof DepTrackingCache) {
           return reader.cacheKeyRoot.lookup(
-            reader.keyMaker.forQuery(query).lookupQuery(query),
+            query,
             contextValue.store,
             fragmentMatcher,
             JSON.stringify(variableValues),
@@ -143,7 +138,7 @@ export class StoreReader {
       }: ExecSelectionSetOptions) {
         if (execContext.contextValue.store instanceof DepTrackingCache) {
           return reader.cacheKeyRoot.lookup(
-            reader.keyMaker.forQuery(execContext.query).lookupSelectionSet(selectionSet),
+            selectionSet,
             execContext.contextValue.store,
             execContext.fragmentMatcher,
             JSON.stringify(execContext.variableValues),