PATCH: Implement value_to_json for single-datum conversion

Hi all

Whenever I try to work with the new json types I trip over the lack of a 
function to escape text to json. The attached patch against master 
provides one by exposing the existing datum_to_json function to SQL. 
I've used the name value_to_json, but I'm not sure it's necessarily the 
right choice of name.

Please consider this for the 9.2 branch as well as to HEAD, as IMO it's 
very important for basic usability of the json functionality. It applies 
to 9.2 fine and passes "make check". I know it's late in the game, but 
it's also a very small change and it's very hard to build up JSON data 
structures other than simple rows or arrays without at minimum a way of 
escaping `text' to json strings.

This feels basic enough that I'm wondering if there's a reason it wasn't 
included from the start, but I don't see any comments in json.c talking 
about anything like this, nor did I find any -hackers discussion about 
it. I suspect it's just an oversight.

As value_to_json directly wraps datum_to_json it actually accepts record 
and array types too. I didn't see any reason to prevent that and force 
the user to instead use row_to_json or array_to_json for those cases. If 
you don't want to accept this, I can provide a wrapper for escape_json 
that only accepts a text argument instead, but I think *some* way to 
escape text to JSON is vital to have in 9.2.

A docs patch will follow shortly if you're happy that this patch is 
reasonable.

--
Craig Ringer