Reduce the IRAM (and heap) usage of I2C code

The I2C code takes a large chunk of IRAM space.  Attempt to reduce the
size of the routines without impacting functionality.

First, remove the `static` classifier on the sda/scl variables in the
event handlers.  The first instructions in the routines overwrite the
last value stored in them, anyway, and their addresses are never taken.

Author: earlephilhower

cores/esp8266/core_esp8266_si2c.cpp

@@ -524,8 +524,8 @@ static void eventTask(ETSEvent *e)
 
 void ICACHE_RAM_ATTR onSclChange(void)
 {
-	static uint8_t sda;
-	static uint8_t scl;
+	uint8_t sda;
+	uint8_t scl;
 
 	sda	= SDA_READ();
 	scl = SCL_READ();
@@ -682,8 +682,8 @@ void ICACHE_RAM_ATTR onSclChange(void)
 
 void ICACHE_RAM_ATTR onSdaChange(void)
 {
-	static uint8_t sda;
-	static uint8_t scl;
+	uint8_t sda;
+	uint8_t scl;
 	sda	= SDA_READ();
 	scl = SCL_READ();