Implement std traits for Multimap and extend it

[pyfisch]

Missing traits:

• ::std::ops::Index (already implemented)
• ::std::fmt::Debug Implement Debug for MultiMap. #1
• ::std::cmp::PartialEq Implement Eq and PartialEq for MultiMap. #3
• ::std::cmp::Eq Implement Eq and PartialEq for MultiMap. #3
• ::std::default::Default
• ::std::iter::IntoIterator Implemented IntoIterator for MultiMap (ref, mut ref and consuming) #4
• ::std::iter::FromIterator
• ::std::iter::Extend

There are more functions not implemented for MultiMap which are implemented for HashMap but they are useful here too. An example is the values function.

C++ also provides a multimap and maybe we can borrow some ideas from them. For example iterating over the values of a key would be useful.

There is also an issue to add a multimap to rust.

[havarnov]

I agree that values could be useful, maybe open a new issue for that? We have to decide if the .values() returns a vector of vectors or just the first value in the value vectors.

[havarnov]

For example iterating over the values of a key would be useful.

Isn't that already implemented for the Vec returned by get_vec()?