migrate fixedsizearray to swift3

[JaapWijnen]

#181

[kelvinlauKL]

I've made a few changes on this:

To keep consistent with the other native Collection types, I've removed the argument labels for append.

removeAtIndex(index: Int) has also been changed to removeAt(index: Int) to avoid mentioning index twice.

Thanks!

[JaapWijnen]

Ah good point! I'll try to think of it next time.