Introduce LibCore and move GElapsedTimer => CElapsedTimer.

I need a layer somewhere between AK (usable both by userspace and kernel)
and LibGUI (usable by userspace except WindowServer.) So here's LibCore.

LibCore/CElapsedTimer.cpp

@@ -0,0 +1,29 @@
+#include <LibCore/CElapsedTimer.h>
+#include <AK/Assertions.h>
+#include <sys/time.h>
+
+void CElapsedTimer::start()
+{
+    m_valid = true;
+    gettimeofday(&m_start_time, nullptr);
+}
+
+inline void timersub(const struct timeval* a, const struct timeval* b, struct timeval* result)
+{
+    result->tv_sec = a->tv_sec - b->tv_sec;
+    result->tv_usec = a->tv_usec - b->tv_usec;
+    if (result->tv_usec < 0) {
+        --result->tv_sec;
+        result->tv_usec += 1000000;
+    }
+}
+
+int CElapsedTimer::elapsed() const
+{
+    ASSERT(is_valid());
+    struct timeval now;
+    gettimeofday(&now, nullptr);
+    struct timeval diff;
+    timersub(&now, &m_start_time, &diff);
+    return diff.tv_sec * 1000 + diff.tv_usec / 1000;
+}