Allow using anonymous functions in operator expressions

[tobil4sk]

This was disabled to deal with #5854, however, with this patch it is possible to avoid that issue without disallowing other types of expressions. For good measure, I have added a few more tests for #5854 to make sure this doesn't break that.

This makes anonymous functions nicer to use with operator overloading which is useful for functional style programming. It also fixes their usage with >>, which tink_core overloads for chaining futures.

See #10782

[Simn]

I love this issue because I can come up with contrived examples where the grammar breaks.

Involuntary array access

class C {
	public function new() {}

	public function call() {
		trace("called!");
	}
}

function main() {
	var f = function() {
		trace("f!");
	}
	[new C()][0].call(); // Array access is not allowed on () -> Void
}

Prefix turned postfix

function main() {
	var i = 0;
	var f = function() {
		trace("f!");
	}
	--i; // Missing ;
}

Abstract - overload

abstract NegativeCounter(Int) {
	public function new() {
		this = 0;
	}

	@:op(-A) inline function decrease() {
		--this;
	}
}

function main() {
	var c = new NegativeCounter();
	var f = function() {
		trace("f!");
	} // () -> Void should be Int
	-c;
}

The first two can be fixed by handling them like the POpen case. The third one might be an acceptable loss.

[Simn]

Here's a weird one just so we can say we considered it:

function main() {
	var f = function() {
		trace("f!");
	}
	<xml></xml> // Unterminated markup literal
}

My opinion on this is if it doesn't break Shiro Games then it's not a problem, so we should check with them before merging.

On a side note, the error seems odd too, I don't know if a plain </xml> should complain about being unterminated...