Use Boost:: (or other) namespace for cmake exports

* The `NAMESPACE` keyword of the [`install( EXPORT )` command](https://github.com/boostorg/hana/blob/6187d448df49db085d4d7ad63d3a5f479594c5cf/CMakeLists.txt#L63) should be used. Targets in namespaces containing `::` can be distinguished from libraries resulting in better error messages (popping up during configuration, instead of linking) if the target was not found.

* Using `Boost::` as namespace would make the resulting target name coherent with the result of `find_package( Boost )` which ~~creates~~ should create an imported target `Boost::hana` (currently, only library targets [are exported as `Boost::`](https://cmake.org/cmake/help/v3.9/module/FindBoost.html))

Thank you for the nice `CMakeLists.txt` ;)

Start in CMakeLists.txt at the install(EXPORT) command linked in the issue, and inspect how exported target names are currently formed. Check CMake's NAMESPACE behavior and the existing Boost:: component naming, then verify that the generated exports use the intended namespace and produce the expected configuration-time diagnostics.

Written by the indexing model from the issue text.