Only /declare/ tags

[martinmoene]

change

#define WV_DEFINE_VALUE_TYPE( type_name, underlying_type ) \
    struct type_name##_tag{}; \
typedef ::wv::whole_value<underlying_type, type_name##_tag> type_name;

to

#define WV_DEFINE_VALUE_TYPE( type_name, underlying_type ) \
typedef ::wv::whole_value<underlying_type, struct type_name##_tag> type_name;

See:

• Jonathan Boccara. Strong types for strong interfaces. Blog post, 8 December 2016.
• Jonathan Boccara. Strong types for strong interfaces. Meeting C++ 2017.