[identity] general cleanup

Summary: i noticed some incorrect patterns in our code using cargo clippy. in particular, we should use ok_or_else instead of ok_or when we're calling a function (lazy evaluation), avoid referencing when the compiler will immediately dereference anyway, and use Box to wrap large enum variants.

Test Plan: cargo build

Reviewers: jon, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8824