[BUGFIX] Reclaiming the memory allocated for static objects *is* necessary.

[BUGFIX] Reclaiming the memory allocated for static objects *is* necessary.

GiNaC allocates memory for static objects (i.e. flyweights, remember tables,
etc), but doesn't free it. This is OK if the program lifetime matches libginac
lifetime, since the OS will reclaim that memory anyway.
However, if the program lifetime is different from that of libginac, this
turns into a memory leak. This happens if someone dlopen's libginac.so, and
dlclose's it later on (read: if someone uses GiNaC via scripting language
bindings).

symbol::autoname_prefix(): there's no need for dynamical memory allocation.
remember_table::remember_tables(): likewise.
function::registered_functions(): likewise.
lib_init::~lib_init(): if library usage count drops to 0, reclaim the memory
                       allocated for flyweights.