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.
Classes reading and writing to the data heap would communicate directly
with the Heap object, and transfer ByteBuffers back and forth with it.
This makes things like caching and locking hard. Therefore all data
persistence activity will be funneled through a Serializer object which
in turn submits it to the Heap.
Introducing this unfortunately resulted in a huge amount of churn, in
which a number of smaller refactorings got caught up as well.
