Add an ``unreachable!()`` macro.

Rationale: having a function which fails means that the location of
failure which is output is that of the unreachable() function, rather
than the caller.

This is part of #8991 but is not all of it; current usage of
``std::util::unreachable()`` must remain so for the moment, until a new
snapshot is made; then I will remove that function entirely in favour of
using this macro.