Merge: Derive all the things

This PR is crazy and inspired by https://github.com/privat/nit/pull/1202#discussion_r26359435
Basically, you will find here some hackish user-level pseudo meta-programming with optional unsafe support from the execution engines trough injection of code in the AST.

The idea is a generalization (and a basic simplification) of the approach or @xymus for serialization.
If fact, there is 2 level of generalizations.

In the compiler, a new phase `deriving` offers a static deriving mechanism. For instance, the annotation `auto_inspect` will implements the `inspect` method with a simple recursive inspection of attributes.

In the standard library, a new module `deriving` offers a general mechanism with a new standard `derive_to_map` method that is expected to dump attributes in a simple HashMap.

This basic low-level method is used to provide user-defined deriving methods.
For instance, the module provide basic derived implementation of `==`, `to_s` and `hash` in pure Nit at the user-level.

Moreover, the compiler phase `deriving` is extended to provide `auto_derive` that statically implements `derive_to_map`

Here an example from the code:

~~~nit
class A
   auto_derive
   super DeriveToS
   var an_int: Int
   var a_string: String
end

var a = new A(5, "five")
assert a.to_s == "an_int:5; a_string:five"
~~~

Pull-Request: #1204
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>

diff --cc lib/standard/collection/abstract_collection.nit
Simple merge