C++2a features

[matter123]

Checklist of C++2a feature syntax support, proposals that do not change syntax are not included.

• Allow lambda-capture [=, this]
• __VA_OPT__
• generic lambdas
• initalizer for bitfield members
• const-qualified pointers to members (X{}.*&X::foo)();
• concepts
• three way comparison (tagging could be better but is acceptable)
• Range-based for statements with initializer
• new attributes
• Allow pack expansion in lambda init-capture
• contracts
• destroying operator delete
• class type non type parameters
• explicit(bool)
• char{8,16,32}_t
• consteval
• nested inline namespaces namespace std::experimental::inline parallelism_v2::execution
• coroutines
• modules