Allow replace existing subroutines with an XS implementations.

--- lib/Dumbbench/Instance/Cmd.pm

+++ lib/Dumbbench/Instance/Cmd.pm

@@ -12,6 +12,7 @@ use Class::XSAccessor {

     command

     dry_run_command

   )],

+  replace => 1,

   accessors => [qw(

     use_shell

   )],

--- lib/Dumbbench/Instance/PerlEval.pm

+++ lib/Dumbbench/Instance/PerlEval.pm

@@ -23,6 +23,7 @@ use Class::XSAccessor {

     code

     dry_run_code

   )],

+  replace => 1,

   accessors => [qw(

     _n_loop_timings

     _n_dry_loop_timings

--- lib/Dumbbench/Instance/PerlSub.pm

+++ lib/Dumbbench/Instance/PerlSub.pm

@@ -24,6 +24,7 @@ use Class::XSAccessor {

     code

     dry_run_code

   )],

+  replace => 1,

   accessors => [qw(

     _n_loop_timings

     _n_dry_loop_timings