Example warning:

cppdap/include/dap/future.h:172:14: error: implicitly-declared ‘dap::ResponseOrError<dap::StackTraceResponse>& dap::ResponseOrError<dap::StackTraceResponse>::operator=(const dap::ResponseOrError<dap::StackTraceResponse>&)’ is deprecated [-Werror=deprecated-copy]

Add missing constructors and assignment operators.
This probably adds more than is absolutely necessary, but there's no
harm in being thorough.