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: https://github.com/encounter/dtk-template/commit/d1334bb79e71af1a7b3b090bffda4adc483f722c)

Resolves #117

objdiff-core/src/diff/code.rs

@@ -41,7 +41,7 @@ pub fn no_diff_code(out: &ProcessCodeResult, symbol_ref: SymbolRef) -> Result<Ob
         });
     }
     resolve_branches(&mut diff);
-    Ok(ObjSymbolDiff { symbol_ref, diff_symbol: None, instructions: diff, match_percent: None })
+    Ok(ObjSymbolDiff { symbol_ref, target_symbol: None, instructions: diff, match_percent: None })
 }
 
 pub fn diff_code(
@@ -67,7 +67,7 @@ pub fn diff_code(
         right.arg_diff = result.right_args_diff;
     }
 
-    let total = left_out.insts.len();
+    let total = left_out.insts.len().max(right_out.insts.len());
     let percent = if diff_state.diff_count >= total {
         0.0
     } else {
@@ -77,13 +77,13 @@ pub fn diff_code(
     Ok((
         ObjSymbolDiff {
             symbol_ref: left_symbol_ref,
-            diff_symbol: Some(right_symbol_ref),
+            target_symbol: Some(right_symbol_ref),
             instructions: left_diff,
             match_percent: Some(percent),
         },
         ObjSymbolDiff {
             symbol_ref: right_symbol_ref,
-            diff_symbol: Some(left_symbol_ref),
+            target_symbol: Some(left_symbol_ref),
             instructions: right_diff,
             match_percent: Some(percent),
         },
@@ -211,7 +211,7 @@ fn arg_eq(
     left_diff: &ObjInsDiff,
     right_diff: &ObjInsDiff,
 ) -> bool {
-    return match left {
+    match left {
         ObjInsArg::PlainText(l) => match right {
             ObjInsArg::PlainText(r) => l == r,
             _ => false,
@@ -236,7 +236,7 @@ fn arg_eq(
             left_diff.branch_to.as_ref().map(|b| b.ins_idx)
                 == right_diff.branch_to.as_ref().map(|b| b.ins_idx)
         }
-    };
+    }
 }
 
 #[derive(Default)]