Summary:
All AST nodes had a member `uid_` that was used as a key in
`SymbolTable`. It is renamed to `symbol_pos_` and it appears only in
`Identifier`, `NamedExpression` and `Aggregation`, since only those types were
used in `SymbolTable`. SymbolGenerator is now responsible for creating symbols
in `SymbolTable` and assigning positions to AST nodes.
Cloning and serialization code is now simpler since there is no need to track
UIDs.
Reviewers: teon.banek
Reviewed By: teon.banek
Subscribers: pullbot
Differential Revision: https://phabricator.memgraph.io/D1836
c03ca5f8f0