Guard against most egregious mistakes in calling the library

LuaJIT FFI is not inherently memory safe and there is no way
to completely guard against the caller doing something that
will trample over memory, but we can get pretty close. Biggest
issue is that an empty table will stand-in for a ref struct with
a null ref. So check for that in all the calls. In the calls that
raise errors we now have a specific error for it. In the other
functions the "nil" object is handled quietly but without a nullptr
dereference and hopefully no UB.

README.md

@@ -11,7 +11,7 @@ Cassowary constraint solving is a technique that is particularly well suited to
 
 There are a few Lua implementations or attempts. The SILE typesetting system has a pure Lua implementation of the original Cassowary code, which appears to be correct but is quite slow. There are two extant Lua ports of Kiwi, one that is based on a C rewrite of Kiwi. However testing of these was not encouraging with either segfaults or incorrect results.
 Since the C++ Kiwi library is well tested and widely used it was simpler to provide a LuaJIT FFI wrapper and use that.
-This package has no dependencies other than a C++11 toolchain to compile the included Kiwi library and a small C wrapper.
+This package has no dependencies other than a C++14 toolchain to compile the included Kiwi library and a small C wrapper.
 
 The Lua API has a pure Lua expression builder. There is of course some overhead to this, however in most cases expression building is infrequent and the underlying structures can be reused.