crimson: introduce assert_moveable().

crimson: introduce assert_moveable().

In C++ `std::moving` a `const`-qualified value yields a constant
r-value reference (`const T&&`) which won't be matched with a callable
taking non-constant r-value reference (like move constructors) but
can play with one taking a constant l-value reference (like copy
constructors do). This behaviour is surprising especially in lambas
where adding or removing the `mutable` specifier may lead to different
behaviour. The problem isn't obvious and it's easy to wrongly drop
the `mutable` druing a clean-up. Therefore introducing a tool for
developers to fail at compile-time if that happens seems desired.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>