GH-219: fix |x| operator int overflow / floating point type inconsistency

When 'x' is an integral arithmetic expression, it's now coerced to
yield a signed integer before taking the absolute value of it to
prevent the common issue of unsigned integer overflow/wraparound for
values below zero.

Using a time or interval value/expression for 'x' now also yields a
time or interval, respective, from the |x| operation instead of
a double.

src/parse.y

@@ -728,7 +728,12 @@ expr:
 	|       '|' expr '|'	%prec '('
 			{
 			set_location(@1, @3);
-			$$ = new SizeExpr($2);
+			auto e = $2;
+
+			if ( IsIntegral(e->Type()->Tag()) )
+				e = new ArithCoerceExpr(e, TYPE_INT);
+
+			$$ = new SizeExpr(e);
 			}
 
 	|       expr TOK_AS type