Throttle to about 1MHz

Run about as many 6502 instructions as can run in .1 seconds running at 1MHz, measure the duration, and then sleep the difference between the duration and .1 seconds of wall clock time.

apple2e.cpp

@@ -1,6 +1,9 @@
 #include <cstdlib>
 #include <string>
 #include <set>
+#include <chrono>
+#include <thread>
+#include <ratio>
 
 using namespace std;
 
@@ -1277,7 +1280,15 @@ int main(int argc, char **argv)
 
     while(1) {
         poll_keyboard();
-        cpu.cycle(bus);
+
+        chrono::time_point<chrono::system_clock> then;
+        for(int i = 0; i < 25575; i++) // ~ 1/10th second
+            cpu.cycle(bus);
+        chrono::time_point<chrono::system_clock> now;
+
+        auto elapsed_millis = chrono::duration_cast<chrono::milliseconds>(now - then);
+        this_thread::sleep_for(chrono::milliseconds(100) - elapsed_millis);
+
         // printf("> ");
         // char line[512];
         // fgets(line, sizeof(line) - 1, stdin);