Permit multiple **kwargs arguments

Given the function call `f(**a, **b)` the meaning is clear and precise. But the best Starlark-compliant alternative I could find is:

```
x = dict(a)
a.update(b)
f(**x)
```

That seems more complex, and it's easy to get it wrong, and it doesn't error if a key is repeated (unlike before).

Given an extension to multiple `**kwargs`, I see no reason not to allow it for `*args` too, although there the argument is weaker.

The issue proposes allowing multiple **kwargs arguments in a Starlark call and raises the possibility of allowing multiple *args arguments too. Start by reviewing the language's call-argument handling and existing duplicate-key behavior; define the semantics and test cases before implementation. Done means the proposal is resolved, specified, and covered by tests.

Written by the indexing model from the issue text.