'foo'[::-1].length(@) should evaluate to 3

[springcomp]

In most implementations, the rhs of a sub-expression is implemented differently when the lhs is a projection.
Due to the shape of the AST, the lhs is actually evaluated on the lhs projection.

However, when slicing a string, the rhs should be treated as a regular sub-expression.

As a result, in most implementations, the following expression does not evaluate correctly:

• 'foo'[::-1].length(@) should evaluate to 3.

In some implementations, the result is ``"oof"ornull`.

Please, consider adding a compliance test.