The Lua/APR binding now contains proper serialization support based on
the excellent table-to-source serializer included in Metalua. Here's
what I've added to / changed about the serialization function:

 - Support for numbers with large precision (e.g. the Metalua
   table-to-source serializer cannot round trip math.pi and
   math.huge without changing the values)

 - Support for Lua functions with upvalues (the Metalua
   table-to-source serializer doesn't support upvalues)

 - Support for userdata objects created by the Lua/APR binding
   (based on a reference mechanism using UUID values)

 - Lots of dedicated tests for the serializer, and of course
   it's also getting tested by the multi threading tests

In the process of adding serialization support I've rewritten the multi
threading tests to take advantage of the new support for upvalues. I've
also removed apr.thread() and renamed apr.thread_create() to
apr.thread() because it now has full support for serialization of
functions (the documentation has been updated accordingly).