Restructure header files.

This should prevent the tests from being recompiled.

2019/src/implementations.hpp

@@ -0,0 +1,9 @@
+#pragma once
+
+#include <iosfwd>
+
+namespace aoc2019 {
+    typedef void (*solution_t)(std::istream &, std::ostream &);
+
+    solution_t get_implementation(int day, bool part2 = false);
+}