mpc_name and mpc_fullname are string literals in correct usage, so they
should really be const instead.

mpc_ops aren't typically const, but the framework shouldn't be doing
anything to clobber it; thus, good to constify it as a reminder.

Switch to using a slightly more semantically correct void ** in the
fastpath bits while we're here, since we only do arithmetic on the outer
layer of pointer and compare the inner to a pointer-typed (NULL).