Method contracts

As discussed on Slack, it would be nice to be able to annotate methods with pre- and post-conditions.

```haxe
@:pre(x >= 3 && x < array.length + 3)
@:post(return != null)
static function getOffset(x:Int):String {
return array[x - 3];
}
```

Potential applications:

- parse the contracts with the static analyser to avoid unnecessary checks (#8815)
- pass the contracts onto an external verifier (Viper, Boogie, Z3…) and emit compilation errors when the code does not fulfil the contract
- parse with dox to slightly enhance documentation

There are also some problems to work out with the syntax, e.g., how to represent:

- the return value? - In the example I used the `return` keyword, which might work.
- the previous value of a variable? - Conventionally annotated with `old`. There might also be issues due to variable shadowing or scoping.
- type assertions? - If a dynamic is passed in?
- enum case assertions?