Custom calling conventions

[Manu]

So I was thinking about this extern(language) thing, the obvious ones are
supported, but it would be really nice to be able to implement custom
conventions for other languages/scripting languages.

For instance, I'm thinking about Android, I have JNI binding code
everywhere, it's really ugly.
I'd love to be able to declare:
  extern(Java) int someJavaFunc(int x, float y)

And then use my function like any regular function, with the 'extern(Java)'
bit handling the JNI business behind the scenes.
I also regularly interact with javascript, lua, C#/mono, and these could
all be implemented the same way.

I'm imaging some mechanism to declare a calling convention (which would be
resolved within the extern(...) statement), and define it with a template,
something like:

callconv Java
{
  R call(T...)
  {
    // process tuple of args, make the call, return something?
  }

  R thisCall(Class, T...)
  {
    // also need a way to implementing methods... this might be enough.
  }
}

Some fancy code in there could conceivably call into any foreign language,
and this would be great!
Now when I: import java.jni;
I have the jni interface, but I also have access to extern(Java), and
that's awesome! :)

The main benefit over using a template, for
instance: jniCall!"functionName"(args...), would be the function name is
not a string, or require custom code construct (facilitating later
re-factoring or delegation to script without changing masses of existing
code, something I have done often), and if it was seen by the language as a
regular function call, you can mark it with all the usual stuff,
const/pure/etc, and apply the expected set of optimisations to the call.

I'm sure this has been discussed before... so go on, tear it apart :)
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