Print class uses String so provide a forward reference in cases where the header hasn't been pulled in.

(the string header also depends upon the print header, so we can't just pull in spring_wiring_string.h since that will have no effect.)
fix for segment fault when converting values to binary. (The buffer size was set assuming a 16-bit integer, not 32-bit.)
fix for converting `INT_MIN` to string, cannot negate in that case because `-INT_MIN` isn't representable in 32-bits.
unit tests