in can't be both a keyword and an identifier

[purpleidea]

I seem to have forgotten to add the following txtar test:

-- main.mcl --
$s = if 42 in [13, -5, 42, 0] {	# make sure the `in` operator works
	"pass"
} else {
	"fail"
}

test $s {}
-- OUTPUT --
Vertex: test[pass]

Woops---

In commit 233625d

lang: parser: Lexer should allow in keyword as a variable name

It turns out I actually broke this. I didn't test properly, sorry!

Is it possible to have both? I don't think most languages allow this, but since we have a $ prefix, it should be easier to disambiguate things.

If so, fixup, if not, make that test failing and add the above test.

[purpleidea]

I've patched this in f92f34d but if there is a better solution I'd love to have it.