Making Contact immutable and Adding a copyWith() method.

[yassinsameh]

A copyWith() method can be useful for cloning an Object or getting a copy while changing one or more fields. I also found it useful when doing Load testing, i needed to have thousands of unique contacts and having the copyWith method was helpful.

[yassinsameh]

Commits include tests added and previous examples/test updated for the new syntax. Was having edge case bugs because of contacts being mutable, think this change makes it much cleaner and makes projects less error prone, please take a look to validate. Great job overall on the package. @joachimvalente

[dphans]

Follow Clean Architecture, I think you no need to do that, instead, custom domain Model (eg. ContactModel) and custom copyWith as you need.