Add indexes to the wiki store to improve performance (#3951)

* First pass at modular wiki indexes

An exploratory experiment

* Fix tests

* Faster checking for existence of index methods

We don't really need to check the type

* Use the index for the has operator

* Fix typo

* Move iterator index methods into indexer modules

Now boot.js doesn't know the core indexers

* Fix up the other iterator index functions

* Fix crash with missing index branch

* Limit the field indexer to values less than 128 characters

* Fallback to the old manual scan if the index method returns null
* Sadly, we can no longe re-use the field indexer to accelerate the `has` operator, because the index now omits tiddlers that have field values longer than the limit

Still need to make the index configuration exposed somehow

* Rearrange tests so that we can test with and without indexers

We also need to expose the list of enabled indexers as a config option

* Test the field indexer with different length fields

So that we test the indexed and non-indexed codepaths

core/modules/startup/load-modules.js

@@ -27,6 +27,7 @@ exports.startup = function() {
 	$tw.Tiddler.fieldModules = $tw.modules.getModulesByTypeAsHashmap("tiddlerfield");
 	$tw.modules.applyMethods("tiddlermethod",$tw.Tiddler.prototype);
 	$tw.modules.applyMethods("wikimethod",$tw.Wiki.prototype);
+	$tw.wiki.addIndexersToWiki();
 	$tw.modules.applyMethods("tiddlerdeserializer",$tw.Wiki.tiddlerDeserializerModules);
 	$tw.macros = $tw.modules.getModulesByTypeAsHashmap("macro");
 	$tw.wiki.initParsers();