MIPS relocation pairing, '$' prefix option & fixes

Implements MIPS relocation pairing logic.
New option for "Register '$' prefix", off by default.
Fixes various regressions introduced by refactoring.

Resolves #122
Resolves #156

objdiff-core/tests/arch_mips.rs

@@ -0,0 +1,19 @@
+use objdiff_core::{diff, obj};
+
+mod common;
+
+#[test]
+#[cfg(feature = "mips")]
+fn read_mips() {
+    let diff_config = diff::DiffObjConfig {
+        mips_register_prefix: true,
+        ..Default::default()
+    };
+    let obj = obj::read::parse(include_object!("data/mips/main.c.o"), &diff_config).unwrap();
+    insta::assert_debug_snapshot!(obj);
+    let symbol_idx = obj.symbols.iter().position(|s| s.name == "ControlEntry").unwrap();
+    let diff = diff::code::no_diff_code(&obj, symbol_idx, &diff_config).unwrap();
+    insta::assert_debug_snapshot!(diff.instruction_rows);
+    let output = common::display_diff(&obj, &diff, symbol_idx, &diff_config);
+    insta::assert_snapshot!(output);
+}