Expose more path fragment operations to public API

[jdimeo]

JsonPath does not create intermediary objects/paths as discussed in #256 , and it shouldn't. However, if you want to do this yourself, very helpful aspects of the JsonPath API are marked as private/protected.

One operation that I was surprised to see was quite difficult was to evaluate a JsonPath at the "immediate parent", something like root -> tail -> previous.

I was able to achieve this using hacky reflection, so this PR opening up the APIs I needed to call with forced reflection access.

1. PathToken.getPathFragment seems like the most obvious- the abstract class marks it as protected but all child implementing classes are public. Not sure why it's protected.
2. PathToken.getPrevoius is already available as prev() but it's package-private. If we're willing to expose getNext why not getPrev?
3. Instantiating intermediary objects through arrays require knowing what index we're on, so ArrayIndexToken.getIndexes gives a mechanism for that without exposing the entire ArrayIndexOperation. This doesn't support slicing, but since it's just a simple getter, it's not promising full support or new functionality.
4. The most controversial one is likely my getPathImpl. This is the entry point to all these helpful path fragment/path token APIs from a JsonPath but I understand how it's confusing with String getPath and might be something you don't want to expose.