Implemented IntoIterator for MultiMap (ref, mut ref and consuming)

[havarnov]

For &MultiMap this implementation makes sense, but for the consuming IntoIterator (for MultiMap) this api will lose the vector of values. So maybe the IntoIterator's should use iter_all() instead.

Either ...

for (key, first_value_in_vec) in multimap {}

... or:

for (key, vector_of_values) in multimap {}