Revert map.each to something which takes two parameters
rather than a tuple.  The current setup iterates over
`BaseIter<(&'self K, &'self V)>` where 'self is a lifetime declared
*in the each method*.  You can't place such a type in
the impl declaration.  The compiler currently allows it,
but this will not be legal under #5656 and I'm pretty sure
it's not sound now.