fix: readonly tuples were recognized as arrays

[Minhir]

Readonly tuples as method arguments were wrongly recognized as arrays.

So get(arg: readonly [number, string]) was converted to readonly number[]. I fixed it. I hope someone can help to find more elegant way to handle this.

[Gerrit0]

Thanks for the PR! I think a cleaner fix will to be to resolve the type reference and check the resolved type rather than relying on the internal method to do that.