abacus: add initial bignum library

src/bignum/bignum.hh

@@ -0,0 +1,20 @@
+#pragma once
+
+#include <vector>
+
+#include <cstdint>
+
+namespace abacus::bignum {
+
+class BigNum {
+public:
+    explicit BigNum(std::int64_t number);
+
+private:
+    void canonicalize();
+
+    std::vector<std::uint8_t> digits_{};
+    int sign_ = 0;
+};
+
+} // namespace abacus::bignum