Investigate serde alternatives

`git-req` doesn't use all of serde -only the barest minimum required for JSON parsing. Investigate switching to something lighter-weight, like https://github.com/not-fl3/nanoserde/

Review git-req's current serde usage and identify the minimum JSON parsing functionality it relies on. Compare that usage with the proposed nanoserde alternative, then determine whether the dependency can be replaced without losing required behavior. Done means the alternative has been evaluated and a replacement decision or implementation is documented.

Written by the indexing model from the issue text.