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.
auto merge of #5796 : nikomatsakis/rust/issue-5656-fix-map-iteration, r=thestinger

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.  It's too bad that maps can't implement `BaseIter` (at least not over a tuple as they do here) but I think it has to be this way for the time being.

r? @thestinger