Fix possible invariant breakage in iter_mut

[mersinvald]

Currently PrimitiveMap::iter_mut returns Option<&mut (Key, Value)>, that makes it possible to mutate the Key and break the invariant leading to failing lookups by a new value of the Key.

Even anyone who is familiar with principals of HashMap data structure won't try to do so, it should be restricted by the API by returning Option<(Key, &mut Value)> from the iterator.

In the current iterators implementation, the native IntoIterators of used underlying data structures are used, so the solution might be in wrapping Buckets into new types and reimplementing the IntoIterator trait for them with more strict mutation constrains.

[mersinvald]

Additionally, the current implementation does not support iterators over BTreeMap bucket, as it is not support invariant leakage :)

[mersinvald]

Probably, this should be postponed until the transition to BST-based Vec bucket implementation #11