Generators: main.py is now dawn_json_generator.py

This merges all the files for main.py together again except
generator_lib.py because there doesn't seeem to be a good way to
separate the pure dawn.json generators and the dawn wire generator.

Also updates the GN templates to make it easier to define new generators
based on generator_lib, and move the Jinja2 template "stdlib" to
generator_lib.py

BUG=dawn:165

Change-Id: I1b5b2ef0a59cb142e214f3af9a58048a88ae949a
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/7880
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>

generator/generator_lib.py

@@ -96,9 +96,25 @@ def _do_renders(renders, template_dir):
     loader = PreprocessingLoader(template_dir)
     env = jinja2.Environment(loader=loader, lstrip_blocks=True, trim_blocks=True, line_comment_prefix='//*')
 
+    def do_assert(expr):
+        assert expr
+        return ''
+
+    def debug(text):
+        print(text)
+
+    base_params = {
+        'enumerate': enumerate,
+        'format': format,
+        'len': len,
+        'debug': debug,
+        'assert': do_assert,
+    }
+
     outputs = []
     for render in renders:
         params = {}
+        params.update(base_params)
         for param_dict in render.params_dicts:
             params.update(param_dict)
         content = env.get_template(render.template).render(**params)