Use ObjectiveSharpie to keep P/Invoke in sync with libsass

[nschonni]

Haven't looked at this seriously, the idea seems interesting http://tirania.org/blog/archive/2017/Jan-18.html
This would probably depend on #53 landing too

[am11]

Interesting. Auto bindings works with simple getter/setter properties and methods. LibSass use cases get trickier with custom imports, custom headers and custom functions implementation where we map anonymous and concrete methods to C function pointers via delegates. I wonder how it would handle those?

I suppose, just like node-sass (static compile) and perl-sass (FFI), each feature of C-API needs hand-rolled implementation with .NET P/Invoke (FFI). Besides, we have already implemented most (if not all) of them, only couple of new / tiny features might be missing.