LibSQL: Resolve cyclic dependency

Previously, class SQL::Key
depends on def class SQL::IndexDef (because inline def index())
depends on def class SQL::KeyPartDef (inline def key_definition())
depends on def class SQL::ColumnDef (because base class)
depends on def class SQL::Relation (because base class)
depends on def class SQL::Key (because inline def hash()).

This hasn't caused any problems so far because Meta.h happened to be
always included after Key.h (in part due to alphabetical ordering).

However, a compilation that for example only contains
    #include <Userland/Libraries/LibSQL/Key.h>
would fail to compile.

This patch resolves this issue by pushing the inline definition of
SQL::Relation::hash() into a different file. Yes, this might reduce
performance marginally, but this gets it to compile again.

Userland/Libraries/LibSQL/Key.h

@@ -8,6 +8,7 @@
 
 #include <AK/RefPtr.h>
 #include <LibSQL/Forward.h>
+#include <LibSQL/Meta.h>
 #include <LibSQL/Tuple.h>
 
 namespace SQL {