ffi purego

[gysddn]

• Added purego the ffi submodule
• Loading libffi.dylib at init of the module
• Replaced all cgo function calls with purego bindings
• Added structs and enums needed for these functions
• Updated some calls to use the new enums
• Used purego callback system for handleClosure
• Removed ffi.m file

This commit leaves some parts of cgo to not break the api but all these parts can be removed by updating the appropriate files using them. cgo, in some places, is exposed to the rest of the api, most notibly the cgo.Handle.