Const a few member variables.

Signed-off-by: Adrian Conlon <adrian.conlon@gmail.com>

M6502/HarteTest_6502/element_t.h

@@ -6,7 +6,7 @@
 
 class element_t {
 private:
-    simdjson::dom::element m_raw;
+    const simdjson::dom::element m_raw;
 
 protected:
     element_t(simdjson::dom::element input) noexcept;

M6502/HarteTest_6502/parser_t.h

@@ -11,7 +11,7 @@ private:
     // Therefore, it can only be used for one document at a time.
     static simdjson::dom::parser m_parser;
 
-    std::string m_path;
+    const std::string m_path;
     simdjson::dom::element m_raw;
 
 public: