Implement unmodifiable Multimaps.

[motlin]

Implement unmodifiable Multimap wrappers like UnmodifiableMutableListMultimap, UnmodifiableMutableSetMultimap, etc.

Implement MutableMultimap<K, V> asUnmodifiable(); in MutableMultimap<K, V> and covariant overrides like MutableListMultimap<K, V> asUnmodifiable(); in MutableListMultimap.

All interfaces that extend MutableMultimap should have an override, including

• MutableBagIterableMultimap
• MutableBagMultimap
• MutableListMultimap
• MutableSetIterableMultimap
• MutableSetMultimap
• MutableSortedBagMultimap
• MutableSortedSetMultimap

The overall design and inheritance hierarchy of unmodifiable MutableMultimaps should be similar to the unmodifiable MutableCollection hierarchy.

[gonzra-bnym]

Hello. I'd like to work on this issue.

[donraab]

Thanks @gonzra-bnym ! I've assigned the issue to you.