[RFA 0/4] Conver observers to C++

[Tom Tromey]

This weekend I decided to try to convert observers to C++.  This
series is the result.

I'm a bit undecided about this patch.  I tend to think it's a mild
improvement, but it does have some disadvantages as well.  Mostly I
covered these in the description for the first patch.

It does have one long(er) term advantage, which is that it's simpler
to add features to this code.  For example, adding a way to provide a
user-data argument to a callback can be done by subclassing the new
gdb_function; or by adding an equivalent to std::bind.

This is broken into a series for easier review, but the patch would
have to be squashed before landing.

This patch requires you to do a clean build after it is applied,
because observer.h now migrates to the source directory.  This could
be fixed by changing the name of the new file -- easy to do if you'd
prefer that.

Built and regtested on x86-64 Fedora 24.

Tom