Simple set_serial_message_callback

[boozook]

Simpler solution with just static boxed fn as trait object. Not perfect because Box+TratiObj gives two derefs. But it easier, without static hashmap (or variadic) and works like update callback. More details and reasons for this in that comment.

How to test it:

# Build, install, run:
cargo playdate run -p=playdate-system --example=set-serial-message-callback --features=sys/lang-items,sys/entry-point --device
--no-read

# Then send messages:
pdtool send msg "one" && pdtool read
pdtool send msg "two" && pdtool read
pdtool send msg "three" && pdtool read
# ...

# Output should be like this:
# serial_message_callback: one
# serial_message_callback: two
# serial_message_callback: three
# ...

[paulyoung]

Thanks!