Avoid double-loop and branching for leap years

This avoids the variable-length array issue n #429. The previous
implementation uses 'leap' to:

    * Run a full 'nyear' loop to populate whether each year is leap.
    * Run a second loop to count the number of days from the array
      (the branch prediction in each iteration will be wrong about
      half the time).

Count the number of days directly in one loop. This is slightly more
memory-efficient and should be more computationally efficient too.

Closes #430. See #429.

src/startofyear.c

@@ -36,27 +36,21 @@ SEXP do_startofyear (SEXP _from, SEXP _to, SEXP _origin)
   int *fromto = INTEGER(_fromto);
 
   int nyear[1] = { (to - from + 1) };
-  int leap[nyear[0]];
-
 
   // generate sequence of dates to work with
   fromto[0] = from;
   for(i=1; i < nyear[0]; i++) {
     fromto[i] = fromto[i-1] + 1;
-  } 
-
-  for(i = 0; i < nyear[0]; i++) {
-    leap[ i ] = ( (fromto[ i ] % 4 == 0 && fromto[ i ] % 100 != 0)
-                 ||
-                   fromto[ i ] % 400 == 0) ? 1 : 0;
   }
 
   for(i=0; i < nyear[0]; i++) {
-    if(leap[i] == 1) { // a leapyear (366 days)
-      fromto[i] = 366;
-    } else {           // a non-leapyear (365 days)
-      fromto[i] = 365;
-    }
+    // A leap year is every year divisible by 4 _except_
+    //   years also divisible by 100. Leap centuries,
+    //   those divisible by 400, also get 366 days.
+    int leap = ( (fromto[ i ] % 4 == 0 && fromto[ i ] % 100 != 0)
+                ||
+                  fromto[ i ] % 400 == 0) ? 1 : 0;
+    fromto[i] = 365 + leap;
   }
 
   /*