libklel

This release reworks the error-reporting APIs for better consistency. It adds interfaces to retrieve additional version information at runtime. It adds several 'abort' functions to the KL-EL standard library that make it possible for expressions to generate runtime errors, which are also useful for testing purposes. The guts of several routines have been relocated to reduce the number of opportunities for memory allocation errors once an expression has been compiled. This release fixes a bug in the interpreter that could cause a crash if the actual argument to a function produces a runtime error.