Add Boost requirement to CMake.
Implement GUID using boost's implementation

Since we're maintaining a C api, the implementation is sometimes less than
intuitive from either a C or C++ perspective. I am trying to use as much
boost as possible while making all the guarantees that the C code makes.

One function was declared deprecated because it "wasn't thread safe". This
was straightforward to repair, and is no longer marked deprecated, and there
are now two ways to convert GUID to String: passing your own character
buffer, and having one returned to you that you need to free.
Rename guid_new to guid_replace

`new` implies some allocation. Since guid_new was actually constructing
a guid in place rather than allocating it, it makes much more sense to
call it guid_replace (or guid_construct). We went with guid_replace.
Cleared up GUID construction and allocation

To allocate a GUID, use guid_malloc. To allocate and construct a guid
(which is actually unique), use guid_new (and be sure to free it using
guid_free).
Corrected uses of guid_to_string.

Previously, guid_to_string had been marked deprecated with a note about
it not being thread-safe. It was much worse than "not thread safe", it
was only safe in a particular situation, and its safety was being violated
throughout the code. It was clear that users of guid_to_string did not
understand what it was purporting to do because of its varied uses. Most
uses simply treated it like a Garbage-Collected Java String (use and forget).
I actually found at least one instance where the string was being freed. (!!!)

I made the method have a particular easy-to-understand semantic: it returns
a pointer to a string which must be freed by the caller. I then tried to
track down all uses of this function and correct them. Mostly, I just changed
the usage to guid_to_string_buff with a stack-allocated string to avoid the
the malloc/free cycle.