Merge: new `with` statement

Merge: new `with` statement

Introduce the `with` statement that is analogous to the `with` of Python or the `uning` of C#.

~~~nit
with x = something do
   x.work
end
~~~

That is equivalent with

~~~nit
do
   var x = something
   x.start
   x.work
   x.finish
end
~~~

Note that an alternative syntax, without the local variable exists:

~~~nit
with something do
   work
end
~~~

that is used when we want some automatic control or whatever. Eg resource allocation or critical section.

~~~nit
do
   var tmp = something
   tmp.start
   work
   tmp.stop
end
~~~

Most real-world examples could be

~~~nit
with file = path.create do
   file.write("Bla")
end
~~~

and

~~~nit
with a_mutex do
   work
end
~~~

## Names

I reused the `finish` method from Iterator.
I introduced the antonym `start` for the other side.

Note that the `start` is important because it helps the object to know it has to prepare itself for a `finish`.

I did not introduce an interface, mainly because I have no idea for a name. C# names it `IDisposable` and the `finish` method is called `Dispose`. There is no `start` because C# designers are lunatic sometime.

In python there is no interface (obviously) and the method are called
`__enter__` and `__exit__` because Python likes identifiers that resemble words eaten by boas.
Note that the returned variable is not the original expression but the result of `__enter__`.
With the Python semantic, but the Nit syntax, the first example will be equivalent to

~~~nit
do
   var tmp = something
   var x = tmp.start
   x.work
   x.finish
end
~~~

I am not sure of the benefits of the additional complexity, but the the more I thing about it the more I think this could be useful, ex for the control case the `start` can return the handler to `finish`.

The issue is that proposed syntax: `with x = something` does not make sense (because `tmp=something` and `x=tmp.start`) thus should be replaced with something else; Python uses `with something as x:` that we can reuse but I do not want because it is ugly.

An other alternative I tough of was to reuse the `for` keyword. but It is convoluted.
The two previous real world examples will then be

~~~nit
for file = path.create do
   file.write("Bla")
end
~~~

and

~~~nit
for a_mutex do
   work
end
~~~

Pull-Request: #1243
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Etienne M. Gagnon <egagnon@j-meg.com>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>