Resolve panics caused by deeper nested orderby

[shahzadlone]

This is an umbrella ticket to tackle panics happening when deeper nesting of ordering by sybtype queries are issues.

There is little chance that #826 might be related.

Here are some documented tests already in the source code (that panic):

• TestOneToManyToOneDeepOrderBySubTypeOfBothDescAndAsc (interface conversion orderby panic #921)
• TestOneToManyToOneWithSumOfDeepOrderBySubTypeAndDeepOrderBySubtypeAscDirections (interface conversion orderby panic #921)
• TestOneToManyToOneWithSumOfDeepOrderBySubTypeOfBothDescAndAsc (Out of range index orderby panic #920)
• TestOneToManyToOneWithSumOfDeepOrderBySubTypeAndDeepOrderBySubtypeOppositeDirections (Out of range index orderby panic #920)

There might be separate reasons for these panics, so make more issues upon investigation to separate them out, for example from the above tests, some might be mapper-related panic, and some related to a nil comparison.

Here is a simple example:
publishers { _sum(authors: {field: age, order: {books: rating DESC}}) { name } }

Test this with requesting children fields up till the leaf subtype node, and also test this without any requested children.

EDIT: As mentioned breaking these out into two sub-tasks (#920 and #921).