Release 1.8.0

• Migrated project to Xcode 11.1
• Ported code to Swift 5.1
• Simplify printed representation of procedures.
• Rewrite of garbage collector: replaced recursive garbage collector with iterative version
• Completed hash functions of library (lispkit hashtable)
• New library: (lispkit comparator)
• Several fixes in (lispkit math): integer->fx renamed to integer->fixnum, introduced fxlogical-shift-right and real->flonum, fixed bit-count, fixed fxmodulo to work with negative numbers
• Statically link libraries. Removed Carthage support.