Support length property for array types

(verbatim from forum)
The problem is fairly obvious. Although an expression of the form arrayExpression.length looks like a field access it is actually a special case. I just have not yet implemented this special case in the rule type checker and interpreter/compiler. All that is needed is to extend the typecheck, interpret and compile methods on class FieldExpression to detect cases where the target of the field access has an array type and do something relevant to that case.