Fix JSONPointer handling of empty path and empty keys

Handling flattening of an empty object as an empty struct (Jeffail#83)

* handling flattening of an empty object as an empty struct

* handlng empty arrays

* adding FlattenIncludeEmpty

* function organization

Ensure no negative indexing

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Edit ArrayConcat comments

Formatting

Expose path to slice functions

Allow append syntax (-) for arrays in set

Allow array element deletion from Delete

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