faster Readonly implementation

 The Readonly module (q.v.) is an effective way to create non-modifiable
 variables. However, it's relatively slow.
 .
 The reason it's slow is that is implements the read-only-ness of variables via
 tied objects. This mechanism is inherently slow. Perl simply has to do a lot
 of work under the hood to make tied variables work.
 .
 This module corrects the speed problem, at least with respect to scalar
 variables. When Readonly::XS is installed, Readonly uses it to access the
 internals of scalar variables. Instead of creating a scalar variable object
 and tying it, Readonly simply flips the SvREADONLY bit in the scalar's FLAGS
 structure.