message evaluation fix

trying to hunt down the bug I was experiencing in the following issue: https://github.com/stevedekorte/io/issues/244 and this seems to be the fix.

I don't really know the codebase so this possibly breaks stuff that I'm unaware of but the tests still pass and it fixes the cases shown in the issue. eg:

    Io> List clone @append(2 * 10)
    ==> list(20)