Use Mustachio AOT compiler for default templates

[srawlins]

This is a pretty big change; here goes:

• Contains the content of Fix runtime renderers rendering of certain sections #2674 and Escape variable renders in runtime renderer #2676
• SidebarGenerator is much simplified; only takes a render function, not a Template object, which is only used in runtime renderers.
• Add blank lines in generated AOT renderer files which correct the whitespace of the output. 800 newlines in one file!
• templates.dart is viciously refactored; the diff is not really meaningful, maybe just new content is meaningful:
  • explicit list of partials is no longer needed; partials are eagerly parsed in both the Runtime renderers and the AOT renderers.
  • TemplatesLoader classes also not needed.
  • Templates class wildly refactored; instead of containing Template objects (only needed by the Runtime renderers; in AOT, the AOT render Dart code is in effect the parsed Template itself), it contains a dozen or so render functions.
  • Templates.fromContext now picks an subtype of Templates based on format ('html', 'md') and on presence of a templatesDir option.
• Dartdoc.generator is made not final purely so that we can test two generators on one Dartdoc; this saves ~20 seconds in testing.
• New renderers_output_test.dart generates HTML docs for a package using the AOT renderers (default) and the runtime renderers (forced), then walks the output trees of each result, comparing them, and making sure they generate the exact same bytes.

[coveralls]

Coverage increased (+8.0%) to 66.06% when pulling 2404875 on srawlins:aot-online into 58f7181 on dart-lang:master.

[jcollins-g]

This is great! Also appreciate the precision in the comparison test, makes me much more comfortable switching this on!