Because UBBR is only 12 bits, we were overflowing it at 300 baud because of the use of the U2X bit. Now we turn off U2X if it would yield a UBBR value that would overflow.
Note that this breaks 300 baud communication with the computer on the Uno and Mega 2560 because the 8U2 USB-serial firmware has this same bug (and previously they cancelled each other out). Since, however, it seems more likely that people will need to use 300 baud to communicate with other (legacy) hardware than with the computer, I'm making this change. Issue for 8U2 firmware bug: http://code.google.com/p/arduino/issues/detail?id=542http://code.google.com/p/arduino/issues/detail?id=522
This makes explicit the String constructors that take numeric types and chars and removes the versions of concat() and operator=() and operator+() that accept numberic types.
It also replaces the operator bool() with a operator that converts to a function pointer. This allows for uses like "if (s)" but not "s + 123". See: http://www.artima.com/cppsource/safebool.html. This allowed removing the disambiguating operator+() functions and relying solely on StringSumHelper and anonymous temporaries once again.
Also, now treating unsigned char's like int when constructing Strings from them, i.e. String(byte(65)) is now "65" not "A". This is consistent with the new behavior of Serial.print(byte).
The operator bool() means that you could implicitly convert a String to a bool and then add it to it an int, for example. Which means our operator+ has to match exactly or it will be ambiguous.
Changing toLowerCase(), toUpperCase(), trim() and replace() to return void instead of a reference to the string that's just been changed. That way, it's clear that the functions modify the string they've been called on.
Now Serial.write() places characters in the transmit buffer, and the data register empty interrupt reads and transmit them. Based loosely on the implementation here: ftp://wookey.org.uk/arduino.
http://code.google.com/p/arduino/issues/detail?id=262
That is, there's now a pins/ directory in a platform, which includes multiple directories, each of which has its own pins_arduino.h. The boards.txt gets a new preferences, <BOARD>.build.pins, whose values is a sub-directory of the pins/ directory (possibly with a "platform:" prefix). That sub-directory is then placed in the include path during compilation.
