Add support for long double

[codemercenary]

Need to ensure that all write serializers support this type.

[gittyupagain]

Should that be sizeof(long double)?

[codemercenary]

For correctness, yes. I'm surprised that the unit test didn't catch this one. I'll add a test that does catch it, then fix this spot.

[codemercenary]

Looks like this wasn't getting caught because sizeof(double) == sizeof(long double) on my particular build, and also, the build server actually does catch this failure case correctly. Easy enough to fix.

[gittyupagain]

In src/leapserial/serial_traits.h, control in the type() function in struct primitive_serial_traits<T...> can reach the end of the function without returning a value when T is long double.

[codemercenary]

Actually there's a lot more to it than that--I have to add a new f80 serial atom type to handle this case. I'll push the change now, you tell me what you think.

[gittyupagain]

Clang warns that you should add explicit braces to avoid a dangling else in serial_traits.h (line 80).

[codemercenary]

Oh clang.

[gittyupagain]

You are missing a case statement for serial_atom::f80 in leap::internal::protobuf::ToWireType.