Modernize syntax

Lot of cleanup and modernization in the syntax of Nit

* depreciate old FFI syntax: `extern` without `class` and non-dotted extern notation (quad or space)
* reduce the usage of old style getters and setters (they will be disabled soon)
* remove the sortcut `=` for one-liner function
* hexadecimal litterals `var x = 0x7F`