revised API and implementation

[hhas]

• significantly improves performance by caching parse method types and wrapper functions on first use
• JS<->ObjC type bridging is fully integrated with ref-napi's JS<->C type descriptors and uses ObjC-style type encoding strings throughout
• redesigned APIs for creating blocks, structs, subclasses, and inout arguments
• ns, js functions support custom handling of unconverted values
• updated example scripts and documentation

TO DO:

• inout arguments in blocks and subclass methods
• array, union, bitwise types
• update test.js
• optionally replace ffi-napi with C++ extension to improve performance
• clean up code comments