Make re-emitting events much more memory efficient

The previous impl bluntly created a closure for every event type
and source emitter we set up a re-emit for. We can do much better
than this fairly easily by having one bound handler for each event
name and moving it into a class so we have one emitter per target,
since 99% of the time the target is the client object.

src/client.js

@@ -40,7 +40,7 @@ const SyncApi = require("./sync");
 const MatrixBaseApis = require("./base-apis");
 const MatrixError = httpApi.MatrixError;
 
-import reEmit from './reemit';
+import ReEmitter from './ReEmitter';
 
 const SCROLLBACK_DELAY_MS = 3000;
 let CRYPTO_ENABLED = false;
@@ -115,6 +115,8 @@ try {
 function MatrixClient(opts) {
     MatrixBaseApis.call(this, opts);
 
+    this.reEmitter = new ReEmitter(this);
+
     this.store = opts.store || new StubStore();
 
     this.deviceId = opts.deviceId || null;
@@ -364,7 +366,7 @@ MatrixClient.prototype.initCrypto = async function() {
         this._cryptoStore,
     );
 
-    reEmit(this, crypto, [
+    this.reEmitter.reEmit(crypto, [
         "crypto.roomKeyRequest",
         "crypto.roomKeyRequestCancellation",
     ]);
@@ -3275,7 +3277,7 @@ function _PojoToMatrixEventMapper(client) {
     function mapper(plainOldJsObject) {
         const event = new MatrixEvent(plainOldJsObject);
         if (event.isEncrypted()) {
-            reEmit(client, event, [
+            client.reEmitter.reEmit(event, [
                 "Event.decrypted",
             ]);
             event.attemptDecryption(client._crypto);