Make tiny twi_reply inline

twi_reply is a chunk of code that can be inlined and actually save IRAM
space because certain conditions acan be statically evaluated by gcc.

Sketch uses 270823 bytes (25%) of program storage space. Maximum is 1044464 bytes.
Global variables use 27944 bytes (34%) of dynamic memory, leaving 53976 bytes for local variables. Maximum is 81920 bytes.

401000cc l     F .text1	00000014 twi_delay
401000f4 g     F .text1	00000052 twi_stop
40100148 g     F .text1	0000003b twi_releaseBus
401001b0 g     F .text1	00000206 twi_onTwipEvent
401003d0 l     F .text1	000001e7 onSdaChange
401005c4 l     F .text1	000002fd onSclChange
401008c4 l     F .text1	0000003b onTimer
40100918 g     F .text1	00000085 millis
401009a0 g     F .text1	0000000f micros
401009b0 g     F .text1	00000022 micros64
401009d8 g     F .text1	00000013 delayMicroseconds
401009f0 g     F .text1	00000034 __digitalRead
401009f0  w    F .text1	00000034 digitalRead
40100a3c g     F .text1	000000e4 interrupt_handler
40100b20 g     F .text1	0000000f vPortFree

0x0000000040107434                _text_end = ABSOLUTE (.)

Author: earlephilhower

cores/esp8266/core_esp8266_si2c.cpp

@@ -350,7 +350,7 @@ void twi_attachSlaveTxEvent( void (*function)(void) )
   twi_onSlaveTransmit = function;
 }
 
-void ICACHE_RAM_ATTR twi_reply(uint8_t ack)
+inline void ICACHE_RAM_ATTR twi_reply(uint8_t ack)
 {
   // transmit master read ready signal, with or without ack
   if (ack) {