[#94] Implement Serde encode/decode for Code

This MR attempts to resolve the suggestion in #94 (closed) relating to being able to serde-encode Code objects such that they can be persisted to files and loaded later as configurations.

To implement the serde traits for Code, it was necessary (unless im missing something obvious) to implement them in all the objects Code contains. This necessitated changes to gilrs-core.

I am marking this MR as a draft for three primary reasons:

1. derive needs to be implemented for platforms other than linux (should be done now.)
2. tests need to be added to ensure this feature actually works (currently have an "example" I used for testing...)
3. I am not sure how to handle the changes made to gilrs-core relating to the additional of a new dependency, or how to keep serde as an optional dependency.