Use a struct to hold globals for TWI

Thanks to the suggestion from @mhightower83, move all global objects
into a struct.  This lets a single base pointer register to be used in
place of constantly reloading the address of each individual variable.

This might be better expressed by moving this to a real C++
implementaion based on a class object (the twi.xxxx would go back to the
old xxx-only naming for vars), but there would then need to be API
wrappers since the functionality is exposed through a plain C API.

Saves 168 additional code bytes, for a grand total of 550 bytes IRAM.

earle@server:~/Arduino/hardware/esp8266com/esp8266/tools$ ./xtensa-lx106-elf/bin/xtensa-lx106-elf-objdump -t  -j .text1 /tmp/arduino_build_849115/*elf | sort -k1 | head -20

401000cc l     F .text1	00000014 twi_delay
401000e8  w    F .text1	00000032 twi_releaseBus
40100128 g     F .text1	00000217 twi_onTwipEvent
4010034c l     F .text1	00000149 onSdaChange
4010049c l     F .text1	00000267 onSclChange
40100704 l     F .text1	00000028 onTimer

Author: earlephilhower