rename send_map to hashmap

This makes the module much more discoverable, and is in line with the
'treemap' naming.
get rid of the std::map::Map LinearMap impl

This legacy trait needs to be dropped in order to migrate to
core::container::Map, which doesn't require @ and Copy, and will make
proper use of borrowed pointers.
rename hashmap find/get -> find_copy/get_copy
rename hashmap find_ref/get_ref -> find/get
add find method to the core::container::Map trait
remove the LinearMap get_copy method

This isn't simpler (or faster) than just doing the copy in the caller,
and it doesn't work for types that can be cloned but not copied.
remove cautionary wording in the hashmap docstring

it's pretty solid, at least relative to most of the other modules
rm unnecessary usage of 'unsafe' from hashmap
improve hashmap/treemap documentation
switch LinearMap to current constructor convention
fix core-map.rs benchmark
fix tests
Merge pull request #4594 from thestinger/map

more work on the map trait and TreeMap/LinearMap