Print serial terminal settings

[midouest]

• Sets a TerminalIO metatable on the term table passed to handler.configure(term) with a :print() method for printing the current settings. This functions similarly to executing stty -F /path/to/tty -a on the command line.
• Fixes a small typo in the cc table docs
• Fixes an off-by-one error when loading the CC values into Lua

Example output of TerminalIO:print():

dev_serial: TTY device Playdate at /dev/ttyACM0 handled by playdate

(before)
speed: 9600 baud
cc: intr=3, quit=28, erase=127, kill=21, eof=4, eol=0, start=17, stop=19, susp=26, min=1, time=0
cflag: -parenb, -parodd, cs8, -hupcl, -cstopb, -cread, -clocal
iflag: -ignbrk, -brkint, -ignpar, -parmrk, -inpck, -istrip, -inlcr, -igncr, -icrnl, -ixon, -ixoff, -iuclc, -ixany, -iutf8
oflag: -opost, -olcuc, -ocrnl, -onlcr, -onocr, -onlret, -ofill, -ofdel, vt0
lflag: -isig, -icanon, -iexten, -echo, -echoe, -echok, -echonl, -noflsh

(after)
speed: 115200 baud
cc: intr=3, quit=28, erase=127, kill=21, eof=4, eol=0, start=17, stop=19, susp=26, min=0, time=5
cflag: -parenb, -parodd, cs8, -hupcl, -cstopb, -cread, -clocal
iflag: -ignbrk, -brkint, -ignpar, -parmrk, -inpck, -istrip, -inlcr, -igncr, icrnl, -ixon, -ixoff, -iuclc, -ixany, -iutf8
oflag: -opost, -olcuc, -ocrnl, -onlcr, -onocr, -onlret, -ofill, -ofdel, vt0
lflag: -isig, -icanon, -iexten, -echo, -echoe, -echok, -echonl, -noflsh

[tlubke]

Feels a bit heavy for what it's doing. A couple of notes, but will think about this a little bit more too.

• TerminalIO isn't named very Lua-like.
• Probably don't need TerminalIO:print() since __tostring() is already set in the metatable.
• In the __tostring() method, I wonder if there is a clever way to make use of the existing tables instead of creating more.
• I think making output closer to the tables/variables in the serial module makes more sense than making output match stty.
• If you want to make a separate PR for the two fixes, I don't see a problem with those being merged right away.