improve ergodox ez performance

With these changes, the ergodox ez goes from 315 scans per second
when no keys are pressed (~3.17ms/scan) to 447 (~2.24ms/scan).

The changes to the pin read are just condensing the logic, and
replacing a lot of conditional operations with a single bitwise
inversion.

The change to row scanning is more significant, and merits
explanation. In general, you can only scan one row of a keyboard
at a time, because if you scan two rows, you no longer know
which row is pulling a given column down. But in the Ergodox
design, this isn't the case; the left hand is controlled by an
I2C-based GPIO expander, and the columns and rows are *completely
separate* electrically from the columns and rows on the right-hand
side.

So simply reading rows in parallel offers two significant
improvements. One is that we no longer need the 30us delay after
each right-hand row, because we're spending more than 30us
communicating with the left hand over i2c. Another is that we're
no longer wastefully sending i2c messages to the left hand
to unselect rows when no rows had actually been selected in the
first place. These delays were, between them, coming out to
nearly 30% of the time spent in each scan.

Signed-off-by: seebs <seebs@seebs.net>

keyboards/ergodox_ez/config.h

@@ -31,6 +31,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 /* key matrix size */
 #define MATRIX_ROWS 14
+#define MATRIX_ROWS_PER_SIDE (MATRIX_ROWS / 2)
 #define MATRIX_COLS 6
 
 #define MOUSEKEY_INTERVAL       20
@@ -80,7 +81,16 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 #define RGBW 1
 
-/* Set 0 if debouncing isn't needed */
+/* "debounce" is measured in keyboard scans. Some users reported
+ * needing values as high as 15, which was at the time around 50ms.
+ * If you don't define it here, the matrix code will default to
+ * 5, which is now closer to 10ms, but still plenty according to
+ * manufacturer specs.
+ *
+ * Default is quite high, because of reports with some production
+ * runs seeming to need it. This may change when configuration for
+ * this is more directly exposed.
+ */
 #define DEBOUNCE    15
 
 #define PREVENT_STUCK_MODIFIERS