Make Reed-Solomon faster by using power of generator representation of

	Make Reed-Solomon faster by using power of generator representation of
	GF(256)*.

	* grub-core/lib/reed_solomon.c (grub_uint16_t) [TEST]: Removed.
	(gf_double_t): Likewise.
	(gf_invert): Removed.
	(gf_powx): New array.
	(gf_powx_inv): Likewise.
	(scratch): Move higher.
	(gf_reduce): Removed.
	(gf_mul): Use powx.
	(gf_invert): Likewise.
	(init_inverts): Replaced with ...
	(init_powx): ...this. All users updated.
	(pol_evaluate): Replace multiplications with additions.
	(rs_encode): Likewise.
	(gauss_eliminate): Call gf_invert.
	(grub_reed_solomon_add_redundancy): Call init_powx.
	(grub_reed_solomon_recover): Call init_powx unconditionally.