Added `CRADLE_SCOPE_GUARD`.

* jamroot: Added `use-project` for `src/scope_guard`.
* cradle/scope_guard/scope_guard.hpp: Newly added.
* src/scope_guard/jamfile: Newly added.
* test/scope_guard/jamfile: Newly added.
* test/scope_guard/scope_guard_run.cpp: Newly added.