Allow EC_GROUP objects to share precomputation for improved memory

efficiency (EC_PRE_COMP objects are now constant once completed).

Extend 'extra_data' API to support arbitrarily many slots (although we
need only one at the moment).

Modify EC internal 'extra_data' API: EC_GROUP_[clear_]free_extra_data
now frees only a single slot (the previous functions are available as
EC_GROUP_[clear_]free_all_extra_data).

Submitted by: Nils Larsch
Reviewed by: Bodo Moeller