Navigation in JSON trees

[michaelhkay]

I propose that the parse-json function should create a pinned tree, so that upwards navigation to parent and ancestor j-nodes becomes possible.

I propose introducing the key specifier .., with $M?.. being a shorthand for ($M => label())?parent, giving a convenient and familiar way to navigate from a j-node to its parent in a pinned tree. For example, $M?..?name gives the value of the name property in the immediately containing map.

Similarly, I propose introducing the key specifier ... to navigate to ancestors, so $M?... becomes a shorthand for ($M => label())?ancestors(), and $M?...?name returns the name property of all containing maps.

For symmetry I suggest we also provide ... as an abbreviated axis step, short for ancestor::node().

I'd like to find a better name for "pinned". Perhaps "tracked" better captures that what it does is to track downward navigation steps and make them reversible.

I'd also like to introduce the terms j-tree and j-node. A j-tree is a map or array, recursively expanded to include its entries or members. A j-node is a value in a j-tree. Perhaps confine the usage to maps and arrays that have been pinned/tracked.

[michaelhkay]

The other label property that's very useful is the key property. `I can't immediately think of nice syntax for getting that. The fn:key() function, of course, is already taken in XSLT.

[michaelhkay]

The .. syntax also gives the ability to detect that we're at the top of the tree using match=".[empty(?..)]" -- something that we observed was lacking in the case-study at #1786 .