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.