This commit is the first in the rewrite of libxpc. It implements the basic API (that is, the XPC object system and the most basic objects and collections). It also adds some tests for the basic API.
The current test suite uses ctest as the testing framework, but for future tests (such as for connections), it may be necessary to find something a little more robust (or roll our own).
Future commits will implement the rewritten connection system based on top of libdispatch's private Mach API, which is made specifically for this purpose (it even has certain methods marked as `4libxpc`).
