Describe Cell width as a narrow/wide boolean.

A character cell can only be either narrow or wide.  It's more
convenient to represent that as an int containing 1 or 2, but slightly
more correct to represent it as a "boolean" single-bit integer.

src/terminal/terminal.cc

@@ -106,7 +106,7 @@ void Emulator::print( const Parser::Print *act )
 
     fb.reset_cell( this_cell );
     this_cell->append( ch );
-    this_cell->set_width( chwidth );
+    this_cell->set_wide( chwidth == 2 ); /* chwidth had better be 1 or 2 here */
     fb.apply_renditions_to_cell( this_cell );
 
     if ( chwidth == 2 ) { /* erase overlapped cell */
@@ -131,7 +131,7 @@ void Emulator::print( const Parser::Print *act )
 	   base character [e.g. start of line], if the
 	   combining character has been cleared with
 	   a sequence like ED ("J") or EL ("K") */
-	assert( combining_cell->get_width() == 1 );
+	assert( !combining_cell->get_wide() );
 	combining_cell->set_fallback( true );
 	fb.ds.move_col( 1, true, true );
       }