Create interfaces for MultiReader collections.

[donraab]

The three MultiReader collections do not currently have interfaces with any specialized APIs. This means you cannot call methods like withReadLockAndDelegate and withWriteLockAndDelegate unless you have the implementation class in your code. This requires you to construct the implementation classes explicitly, and means you cannot use the factory methods for creating the classes.

MutableList<Integer> list = Lists.multiReader.with(1, 2, 3);
// no access to withReadLockAndDelegate or withWriteLockAndDelegate

vs.

MultiReaderFastList<Integer> list = MultiReaderFastList.newListWith(1, 2, 3);
// access to withReadLockAndDelegate or withWriteLockAndDelegate

[donraab]

Fixed by @canthonyl in #719