abacus: add parse library

Would need some improvements, such as by using the C++ Flex scanner
interface.

It will also need to start using the BigNum once it has been
implemented.

src/parse/parser-driver.hh

@@ -0,0 +1,30 @@
+#pragma once
+
+#include <string>
+
+#include "parser.hh"
+
+namespace abacus::parser {
+
+class ParserDriver {
+public:
+    ParserDriver();
+
+    int parse(std::string filename);
+
+    void scan_open();
+    void scan_close();
+
+    yy::location& location();
+    yy::location const& location() const;
+
+private:
+    // FIXME: will become BigNum
+    int result_ = 0;
+    std::string filename_{};
+    yy::location current_location_{};
+    bool parse_trace_p_ = false;
+    bool scan_trace_p_ = false;
+};
+
+} // namespace abacus::parser