Add an option autoload to PROXYSUBS, to generate an AUTOLOAD subroutine.

Add an option autoload to PROXYSUBS, to generate an AUTOLOAD subroutine.

Like croak_on_error, this is only useful with the Proxy Constant Subroutine
code, as that adds all known constants to the symbol table at compile time.
The (obvious) additional restriction is that directly implementing AUTOLOAD
is only useful if the package wishes to perform no other autoloading, and to
treat all autoload requests as (failed) constant lookups (with errors describing
them as such).