Construct a fresh scope within this scope, with same owner,
which shares its table with the outer scope. Used in connection with
method leave if scope access is stack-like in order to avoid allocation
of fresh tables.
Return the entry associated with given name, starting in
this scope and proceeding outwards. If no entry was found,
return the sentinel, which is characterized by having a null in
both its scope and sym fields, whereas both fields are non-null
for regular entries.