PeekChar1
Implements ISO's peek_char/1: unifies the argument with the next character of the current input stream (see IOPrimitiveUtils.currentInputChannel) without consuming it (so a subsequent GetChar1/Read1 sees the same character again), as end_of_file at end of stream.
Throws
(in_character) if the argument is bound to something other than end_of_file or a one-character atom.
Fails, rather than erroring, if the channel is closed.