Add symbol mapping feature (#118)

This allows users to "map" (or "link") symbols with different names so that they can be compared without having to update either the target or base objects. Symbol mappings are persisted in objdiff.json, so generators will need to ensure that they're preserved when updating. (Example: d1334bb79e)

Resolves #117

objdiff-gui/src/views/mod.rs

@@ -1,6 +1,7 @@
 use egui::{text::LayoutJob, Color32, FontId, TextFormat};
 
 pub(crate) mod appearance;
+pub(crate) mod column_layout;
 pub(crate) mod config;
 pub(crate) mod data_diff;
 pub(crate) mod debug;