implement container::Container for SmallIntMap
drop the StdMap implementation from SmallIntMap
clean up SmallIntMap tests a bit
implement container::Mutable for SmallIntMap
move smallintmap to oldsmallintmap
copy oldsmallintmap.rs to smallintmap.rs
modernize smallintmap

* switch to explicit self
* get rid of the @ box
* replace DVec with ~[] (to get rid of the mutable field)
* implement the new container::Map trait
Merge pull request #4733 from thestinger/smallintmap

modernize smallintmap module