Support for index operator on list expressions

[angrykoala]

There is no way of generating the following Cypher code with the CypherBuilder, as the index operator is only available on variables

Cypher.collect(var)[0]

[angrykoala]

There are 2 approaches to this:

• Make Expr a class with an index method (this is probably breaking)
• Add an index function that accepts an Expr and index number