Require array access index arguments to be given in order

[johnstairs]

Based on feedback, we now require array access index arguments to be given in the correct order, even when dimension names are used.

For example, this would no longer be valid:

MyRec: !record
  fields:
    arrayField: !array
        items: int
        dimensions: [x, y]
  computedFields:
    accessArrayElementByName: arrayField[y:1, x:0]

Note that the x and y arguments are provided out of order. You will now have to write arrayField[x:0, y:1]

Addresses #20