Add CET function identification to symbol listing functionality

[tnballo]

As a follow-on after #16 is completed, or just tackled separately beforehand:

• Re-implement (and cite in README.md) the work of Kim et. al. from "How'd Security Benefit Reverse Engineers? The Implication of Intel CET on Function Identification" (DSN 2022)

• Identified functions should be listed in the --symbols output (and tagged as CET-derived)

• CET-based symbol population can be part of the public xgadget API, to be leveraged by external crates/tools

Motivation: Intel CET's Indirect Branch Tracking (IBT) is prototype-insensitive, it limits but does not prevent JOP