All predicatesShow -- Pretty Print Prolog terms

This module is a first start of what should become a full-featured pretty printer for Prolog terms with many options and parameters. Eventually, it should replace portray_clause/1 and various other special-purpose predicates.

To be done
- This is just a quicky. We need proper handling of portray/1, avoid printing very long terms multiple times, spacing (around operators), etc.
- Use a record for the option-processing.
- The current approach is far too simple, often resulting in illegal terms.
Source print_term(+Term, +Options) is det
Pretty print a Prolog term. The following options are processed:
Define the output stream. Default is user_output
Width of a line. Default is 72 characters.
Left margin for continuation lines. Default is 0.
Distance between tab-stops. Default is 8 characters.
Defines how arguments of compound terms are placed. Defined values are:
Simply place them left to right (no line-breaks)
Place them vertically, aligned with the open bracket (not implemented)
auto (default)
As horizontal if line-width is not exceeded, vertical otherwise.
An integer
Place them vertically aligned, <N> spaces to the right of the beginning of the head.
This is the inverse of the write_term/3 option ignore_ops. Default is to respect them.
List of options passed to write_term/3 for terms that are not further processed. Default:
    [ numbervars(true),
Source prepare_term(+Term, -Template, -Cycles, -Constraints)[private]
Prepare a term, possibly holding cycles and constraints for printing.
Source indent(+Out, +Indent, +Options)[private]
Newline and indent to the indicated column. Respects the option tab_width. Default is 8. If the tab-width equals zero, indentation is emitted using spaces.
Source print_width(+Term, -W, +Options) is det[private]
Width required when printing `normally' left-to-right.
Source pprint(+Term, +Context, +Options)[private]
The bottom-line print-routine.