Reduce overhead of modifying a value with Collections

[alicederyn]

Currently, if Guava is present, Builder.from(value) makes a mutable copy of every collection in value, and .build() then creates immutable copies of them all. If the collection isn't changed, there is no need for a mutable copy, and the two values should share the immutable instances.

This issue does not cover behaviour when Guava is not available.

Collection types affected:

• List
• Set
• Map
• Multiset
• ListMultimap
• SetMultimap