Generate the definitions for magic vtables from data in a regen script.

Generate the definitions for magic vtables from data in a regen script.

Previously perl.h contained a long section of MGVTBL_SET() macros declaring
the core's various magic vtables. Convert the information into data structures
in a new script regen/mg_table.pl, and use this to generate a new file
mg_vtable.h, included by perl.h

This is the first step in reducing the number of places that data relating to
magic vtables is declared (and has to be kept in sync), and will allow more
flexibility in parts of the core's implementation.