hash: add prehashed version cn_slow_hash_prehashed
slow-hash: let cn_slow_hash take 4th parameter for deciding prehashed or not
slow-hash: add support for prehashed version for the other 3 platforms
Keep the immediate direct deps at the library that depends on them,
declare deps as PUBLIC so that targets that link against that library
get the library's deps as transitive deps.
Break dep cycle between blockchain_db <-> crytonote_core.
No code refactoring, just hide cycle from cmake so that
it doesn't complain (cycles are allowed only between
static libs, not shared libs).
This is in preparation for supproting BUILD_SHARED_LIBS cmake
built-in option for building internal libs as shared.
