The example/tap.cpp bug was actually interesting, so I'll document it
here. We were using `tap` with a function that performed side effects.
However, we relied on the side effects being done in a particular order
for the example to pass. This is a violation of what the documentation
says we're allowed to.
On Windows, the call convention is such that arguments are evaluated
from right to left, instead of left to right like on OS X and Linux.
Since `chain` uses `transform` and `transform` expands `f(x)...` inside
a function call, our function with side effects was being called in
reverse order as what we would have expected!
Morale: Listen to the documentation, or perish.
- Allow specializing models_impl with a forward declaration only
- Modularize remaining concept headers
- Include <boost/hana.hpp> in unit tests to ease the transition
- Add the core/dispatch.hpp header to reduce the number of boilerplate includes
