Clean traits & Proc Macros

[contagon]

This is a much too large PR for cleaning up a lot of the internal traits and serialization of factrs.

Specifically,

• A new proc-macro factrs::mark that handle tagging and registering objects for serialization via serde/typetag as well as implementing Residual for residuals
  • Essentially this replaces impl_residual and all the tag_* decl-macros. It should be more robust and easier to use.
• Removed all the TraitSafe traits so traits are Boxed directly instead of via a helper trait
• Added clippy::unwrap_used and replaced most unwraps with expects.
• Added tests for custom Variables, NoiseModels, RobustCosts, and Residuals.