Move lab 1 into a single directory and clean up

- WinAVR makefile is replaced by a much shorter one
- Files related to the lab writeup are deleted
- Lab 1 exercises are in one directory now (no ex#/ directories)
- ntohex(), rxbyte(), txbyte(), and txstring() are split into separate files
Move lab 2 into a single directory
Don't treat *.out files as intermediate files
Ignore *.out files
Move lab 3 into a single directory
Put getanalog() and uart_putchar() in their own files
Setup I/O streams at compile-time instead of run-time

Use FDEV_SETUP_STREAM() to initialize stdout instead of fdevopen(); fdevopen()
requires linking in malloc() from avr-libc while FDEV_SETUP_STREAM() is a
static initializer.
Move lab 4 into a single directory
Factor uart_putchar() and write() into separate files
Setup I/O streams at compile-time instead of run-time

Use FDEV_SETUP_STREAM() to initialize stdout instead of fdevopen(); fdevopen()
requires linking in malloc() from avr-libc while FDEV_SETUP_STREAM() is a
static initializer.
Change main() to return int