Forward @:nativeTypeConstraints in closures to class filter

[kLabz]

I have some issues with native cs libs using type parameter constraints: while I can generally use them with @:nativeTypeConstraints (see #3526), the closures to class filter generates classes without these type constraints, resulting in a successful haxe compilation but a compilation error in C#.

This PR does two things:

• forward @:nativeTypeConstraints meta to these classes
• add type parameter constraints to the needed type parameters

Do not hesitate to tell me if my code doesn't make sense or if there are better ways to do it; I'm still not familiar with ocaml / haxe sources.

Related question: should @:nativeTypeConstraints become a proper meta? (I could add it to this PR)

[nadako]

I am not really familiar (anymore) with this code, and I can't say I fully understand what's going on here, but forwarding constraints makes sense to me and since the tests still pass, it should be fine ;-)

Could you maybe add a test for this somehow?

[kLabz]

Could you maybe add a test for this somehow?

Sure, I'll look into this :)

[kLabz]

I actually dropped the @:nativeTypeConstraints custom meta and used @:nativeGen directly instead. Issues impacted: #3526 #4002 and potentially hxcs#37.

I'll add some tests, trying to reflect what was happening in my application.

[kLabz]

Tests have been added and CI passed.

The code related to metadata has been rewritten, you might want to review again.

[kLabz]

@Simn could this be considered for 4.0?

We would like to switch to official haxe 4.0 when it is available and possibly stick with it for a while for production apps, but our backend cannot work without this fix (or something similar).

I can work on this again as much as needed.

Edit: just rebased haxe 4 rc2 to get an updated build from appveyor (hopefully) before monday.