Optimize pressedKeyswitchCount()

Instead of manually counting set bits, use gcc's `__builtin_popcount()` for a -
supposedly - more efficient count.

Also initialize the counter to zero, while there.

Fixes #11.

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

src/kaleidoscope/hardware/ErgoDox.cpp

@@ -235,15 +235,10 @@ bool ErgoDox::isKeyswitchPressed(uint8_t keyIndex) {
 }
 
 uint8_t ErgoDox::pressedKeyswitchCount() {
-  uint8_t count;
+  uint8_t count = 0;
 
   for (uint8_t r = 0; r < ROWS; r++) {
-    if (!keyState_[r])
-      continue;
-
-    for (uint8_t c = 0; c < COLS; c++) {
-      count += bitRead(keyState_[r], c);
-    }
+    count += __builtin_popcount(keyState_[r]);
   }
   return count;
 }