Merge: Generalize instance creation service so FFI can use it

Move up instance creation from ANew to AbstractCompilerVisitor so they can be used from the FFI implementation. Allows to call extern constructors of extern classes from extern code, and fix #1145.

Pull-Request: #1150
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Jean Privat <jean@pryen.org>

diff --cc src/compiler/abstract_compiler.nit
Simple merge