Automatically detect console width

This can now be overridden with the :consolewidth command at the REPL or
in an init script.
Correctly pretty-print no-argument Nil

Previously, when the Nil constructor took no arguments (as was the case
with HVect), it was not shown in sugared syntax in the pretty-printer.

Fixes #820.
Merge pull request #834 from david-christiansen/semantic-pprint

Automatically detect console width + fix pretty-printing of no-argument Nil constructors