[RFA v3] Convert observers to C++

[Pedro Alves]

Hi Tom,

On 03/10/2018 05:03 AM, Tom Tromey wrote:
> This converts observers from using a special source-generating script
> to be plain C++.  This version of the patch takes advantage of C++11
> by using std::function and variadic templates; incorporates Pedro's
> patches; and renames the header file to "observable.h" (this change
> eliminates the need for a clean rebuild).
> 
> Note that Pedro's patches used a template lambda in tui-hooks.c, but
> this failed to compile on some buildbot instances (presumably due to
> differing C++ versions); I replaced this with an ordinary template
> function.
> 
> Regression tested on the buildbot.

This version LGTM.

Thanks,
Pedro Alves