gentmpl: misc improvements, set up templates for nesting
gentmpl: misc improvements, set up templates for nesting

• Instead of calling template.New(…).Parse(…) separately for each
  template, gentmpl now creates an init function which parses each
  template into the template variable Templates.
  Consequently, users of embedded template files must now use
  Templates.ExecuteTemplate(…, "name", …) instead of nameTpl.Execute(…)

• In case a template file cannot be represented as a backquoted string,
  gentmpl now falls back to fmt.Fprintf("%#v", …).

• gentmpl now uses a template for the output code instead of lots of
  calls to fmt.Fprintf

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