Optimize handling of `JsonPointer.head()`

[cowtowncoder]

(note: this is a placeholder for reported but not verified performance concern)

Looks like implementation of JsonPointer.head() could be problematic for deeply nested pointer instances: and while this is unlikely to be of general security concern, it could be problematic for legitimate use. And as per comments in class, optimization already considered earlier... so let's see how easy it's to do.