core: command (proc) names may now contained embedded nulls

The hash table used to store commands now uses Jim_Obj keys rather
than allocated char *, so embedded nulls are supported.

This means that some API function such as Jim_RenameCommand()
now take Jim_Obj * rather than const char *, however Jim_CreateCommand()
is retained with const char * for convenience and the new Jim_CreateCommandObj()
is added.

This is generally a performance win as the existing Jim_Obj can be used
as the key.

Signed-off-by: Steve Bennett <steveb@workware.net.au>