Store the BM table in mg_ptr instead of after SvCUR().

Store the BM table in mg_ptr instead of after SvCUR().

Previously the 256 byte Boyer-Moore table was stored in the buffer of SvPVX()
after the raw string by extending the buffer.

Given that the scalar is alway upgraded to add PERL_MAGIC_bm magic, to clear
the table and other flags, there's no extra memory cost in using mg_ptr in the
MAGIC struct to point directly to the table.

I believe that this removes the last place in the core that stores data beyond
SvCUR().