Tracking _written was required on AVR devices to work around a hardware
limitation when implementing flush(). The ESP8266 doesn't have the same
issue, so we can remove the tracking and make write() more lightweight.
The cost is a minor increase in work done in flush() when write() was not
previously called, but this should be much rarer than individual character
writes.
This includes empty stubs for most core Arduino functions.
Need to actually implement all those digital reads writes whatever.
Need to prebuild toolchains (xtensa-elf-lx106) for 3 platforms and put them on some download server.
Need to do the same with esptool.
Need to fix 0x40000 binary generation and add correct upload commands.
Maybe even implement uploads over WiFi.
