Make visitor reproduce groups

[hvub]

This modifies the visitor so that is correctly reproduces the groups of clauses in a Cypher query.

For the example

MATCH (a) WITH a MATCH (b) SET a.prop = 'match 2' WITH a, b MATCH (c) WITH a, b, c MATCH (d) RETURN *

the modified visitor outputs

MultiPart [
  Group [
    1: MATCH (a)
    2: WITH a
  ]
  Group [
    3: MATCH (b)
    4: SET a.prop = 'match 2'
    5: WITH a, b
  ]
  Group [
    6: MATCH (c)
    7: WITH a, b, c
  ]
  Group [
    8: MATCH (d)
    9: RETURN *
  ]
]

In essence, the grammar tells us a group is terminated by a WITH or a RETURN. Hence, we can simply collect clauses until a WITH or a RETURN appears. When we visit a WITH or a RETURN, we wrap the current group and start a new group.

Note that it is important to call the overridden implementation of the base visitor, e.g.

super.visitOC_MultiPartQuery(ctx);

or directly do what the base visitor does, i.e.

visitChildren(ctx);

so that the visitor continues further down the parse tree.