PGFService.hs: add web API function "abstrjson"

Abstract syntax trees are represented as strings in the web API. To make them
easier to manipulate in JavaScript, the new function converts them to JSON.

To support structural editing, the nodes are numbered in the same way as in
the bracketed string created when linearizing an abstract syntax tree.

Example: "Pred (That Fish) Fresh" is converted to

	{fun:"Pred",fid:3,
	 children:[{fun:"That",fid:1,
		    children:[{fun:"Fish",fid:0}]},
		   {fun:"Fresh",fid:2}]}

0 files changed, 0 insertions, 0 deletions