Add perfect forwarding in Maybe
Benchmark compile-time implications of using std::forward all around
Add perfect forwarding in some functional utilities
Make BOOST_HANA_RUNTIME_ASSERT print to stderr instead of stdout
refactor .gitignore

- ignore build/, release/ and debug/ directories only if they are
at the root of the project
- ignore build (at the root) not only if it is a directory, but also
a symlink (or any other kind of file, but that shouldn't happen)