imp: qweb compiler now condenses whitespaces

This is a pretty big breaking change: the html output by qweb is now
different.

The main goal is to optimize the compiled templates:
- condense all consecutive whitespaces into a single spaces
- if possible, drop completely some text nodes, based on the following
  heuristic: if a text node is only composed of whitespaces, and
contains at least one linebreak, then it can be dropped.

This leads in my benchmark test to an improvement of about 10%, in
rendering speed and in memory consuption.

Note: whitespace here means anything that matches the \s regexp: newlines,
tabs, ...

close #8

src/qweb.ts

@@ -32,6 +32,9 @@ const DISABLED_TAGS = [
   "optgroup"
 ];
 
+const lineBreakRE = /[\r\n]/;
+const whitespaceRE = /\s+/g;
+
 //------------------------------------------------------------------------------
 // Compilation Context
 //------------------------------------------------------------------------------
@@ -49,6 +52,7 @@ export class Context {
   shouldDefineOwner: boolean = false;
   shouldProtectContext: boolean = false;
   inLoop: boolean = false;
+  inPreTag: boolean = false;
 
   constructor() {
     this.rootContext = this;
@@ -84,6 +88,12 @@ export class Context {
     return newContext;
   }
 
+  withInPre(): Context {
+    const newContext = Object.create(this);
+    newContext.inPreTag = true;
+    return newContext;
+  }
+
   withCaller(node: Element): Context {
     const newContext = Object.create(this);
     newContext.caller = node;
@@ -336,6 +346,12 @@ export class QWeb {
     if (!(node instanceof Element)) {
       // this is a text node, there are no directive to apply
       let text = node.textContent!;
+      if (!ctx.inPreTag) {
+        if (lineBreakRE.test(text) && !text.trim()) {
+          return;
+        }
+        text = text.replace(whitespaceRE, " ");
+      }
       if (ctx.parentNode) {
         ctx.addLine(`c${ctx.parentNode}.push({text: \`${text}\`});`);
       } else {
@@ -410,6 +426,9 @@ export class QWeb {
         }
       }
     }
+    if (node.nodeName === "pre") {
+      ctx = ctx.withInPre();
+    }
 
     this._compileChildren(node, ctx);