Refactored expression printing

Last change on this for now, I promise! :-)

Changes:
* Removed ```show_indented``` and ```default_show_quoted```
* Simplified printing logic:
  * ```print```ing an ```Expr``` or ```QuoteNode``` calls ```show_unquoted(ex)```
  * ```show```ing an ```Expr``` or ```QuoteNode``` calls ```show_unquoted(QuoteNode(ex))```