mk: Remove CFG_INSTALL_SNAP logic. #2664
smallint{map,set}: replace iterators with DoubleEndedIterator

Also changed the tests to iterate over sparse maps/sets.
Add tests for issues which can be closed

Closes #5275, $4547
auto merge of #7913 : brson/rust/rm-install-snap, r=graydon
auto merge of #7921 : bytewiseand/rust/smallint-iter, r=huonw

Made the `iter` and `mut_iter` methods on SmallIntMap and SmallIntSet return double-ended-iterators. These iterators now implement `size_hint`.

Also the iterator tests only tested dense maps/sets, which aren't very useful. So they were changed to iterate over sparse maps/sets.

Fixes #7721
auto merge of #7923 : alexcrichton/rust/add-tests, r=graydon

Closes #5275, #4547
Eliminated SmallIntSet

SmallIntSet is equivalent to BitvSet but with 8 times the memory
overhead. There's no reason for it to exist.