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/Meta.cpp

@@ -10,6 +10,11 @@
 
 namespace SQL {
 
+u32 Relation::hash() const
+{
+    return key().hash();
+}
+
 SchemaDef::SchemaDef(String name)
     : Relation(move(name))
 {