Add index-swap, fix assert in array-append

[gambiteer]

These changes are untested.

[ashinn]

Thanks!

[gambiteer]

I noticed that some arguments were protected by asserts, and others were not.
Do you want argument checks for all arguments?

[ashinn]

Yes, in general there should be more checks. This is still a WIP but I hope to wrap up soon. Notably array-block is not implemented, and the signature of make-specialized-array is incorrect (and probably a few other details).