Added serde feature

[JonathanWoollett-Light]

Summary of the PR

Added serde support via a feature that derives Serialize and Deserialize for IdAllocator and AddressAllocator (and contained types). This should in the future allow exact restoration of IdAllocator and AddressAllocator from a stored version. I would suggest it is just best practice to derive Serialize and Deserialize for crate public types (like Debug). This change should have no affect (including compile time) to the existing code base.

You may wish to change the documentation to build with cargo rustdoc --all-features but I was unsure what changes to make here and how to annotate them.

Requirements

Before submitting your PR, please make sure you addressed the following
requirements:

• All commits in this PR are signed (with git commit -s), and the commit
  message has max 60 characters for the summary and max 75 characters for each
  description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• Any newly added unsafe code is properly documented.