Implement efficient map iter, fix test bug

[fhackett]

Map iteration now uses the reflect package to avoid the alternatives of allocating potentially large slices of keys or strange goroutine trickery which we discussed IRL.

There were iterator tests already, so I kept them as-is.

I did notice a strange test failure due to iterator objects getting re-used across two separate tests, so I fixed that by thunking iterator instantiation, wrapping the iterator construction in parameterless functions.