ddvisitor v0.9.6

Add `identity` and `noop_const` helpers

`identity<T>` can be used as a static type filter inside `serial_visitor`
`noop_const<V>` is used where a constant `V` need to be always returned (complements `noop_bool`)

ddvisitor v0.9.4

Another fix of CMake tpack discovery when build via `FetchContent`

ddvisitor v0.9.3

Configure successfully if tpack has already been fetched and made ava…

ddvisitor v0.9.2

Add forgotten ddvisitor-config.cmake.in template

ddvisitor v0.9.1

Fix CMake install and export targets

Double-dispatch visitor v0.9.0

Initial release with most functionality implemented.