There are instantiations of binary_search() used in sstables.cc, but
defined in partition.cc. The instantiations are explicitly declared in
partition.cc, but the types changed and they became obsolete. The
thing worked because partition.cc also instantiated it with the right
type. But after that code will be removed, it no longer would, and we
would get a linker error. To avoid such problems, define
binary_search() in a header.
