sqlparser: Drop obsolete list vars from GenerateQuery
sqlparser: Introduce LIST_ARG token.
sqlparser: split row & column tuples

Row and column tuples are acquiring different semantic
meanings. This is in preparation to allow lists as
bind vars. We're only going to allow lists for column
tuples. There's not much benefit to supporting list
bind vors for row tuples.
sqlparser: Change WriteArg protocol.

Changing WriteArg to accept the full bind var including ':'.
This way, we won't have to special case list vars that start
with "::".
sqlparser: add LIST_ARG to grammar and AST.