Rename: NullsView to BoolView

Rename NullsView to BoolView since this can be used for creating views
of Bool vectors as well. In addition have all an false value views of
a boolean vector return a non-nil vector. When dealing with boolean values
there's a difference between a false values and a nil vector.

Also have NewView of a boolean vector create a new boolean vector.

runtime/vam/expr/arith.go

@@ -63,7 +63,9 @@ func (a *Arith) eval(vecs ...vector.Any) (out vector.Any) {
 			}
 		}()
 	}
-	return f(lhs, rhs)
+	out = f(lhs, rhs)
+	vector.SetNulls(out, vector.Or(vector.NullsOf(lhs), vector.NullsOf(rhs)))
+	return out
 }
 
 func (a *Arith) evalDivideByZero(kind vector.Kind, lhs, rhs vector.Any) vector.Any {