GetCode1
Implements ISO's get_code/1: reads (consuming it) the next character code from the current input stream (see IOPrimitiveUtils.currentInputChannel) and unifies it with the argument, as -1 at end of stream.
Throws
if the argument is bound but not an integer.
(character_code) if it is an integer outside the representable character-code range.
Fails, rather than erroring, if the channel is closed.