Fail in `extern_`'s `retrieve` method.
Support executing non-const Fusion environments.
Make some default intrinsics work on references:

Before trying to fetch the nested value of the trait,
all references are stripped from the type.

These traits were impacted:
* default_implementation_of
* dependencies_of
* name_of
Move the captain's log to GitHub issues.
Minor usability improvement in `detail::bind`.

By using `mpl::map<>::type` instead of `mpl::map<>`, the name
of the template specializations are shorter and error messages
are easier to debug.