Use `signed char` instead of `char` for signed 8-bit integers.
Use `signed char` instead of `char` for signed 8-bit integers.

The C standard allows for `char` to be unsigned. Closes Issue 27.

aungzanbaw/gjstest,aungzanbaw/gjstest,google/gjstest,google/gjstest,aungzanbaw/gjstest,aungzanbaw/gjstest,google/gjstest,google/gjstest