LibCrypto: Add naive implementation of {Un,}SignedBigInteger::to_double

Author: https://github.com/IdanHo
Commit: bcdad57670
Pull-request: https://github.com/SerenityOS/serenity/pull/9859
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibCrypto/BigInt/SignedBigInteger.h

@@ -66,6 +66,7 @@ public:
     String to_base(u16 N) const;
 
     u64 to_u64() const;
+    double to_double() const;
 
     const UnsignedBigInteger& unsigned_value() const { return m_unsigned_data; }
     const Vector<u32, STARTING_WORD_SIZE> words() const { return m_unsigned_data.words(); }