adds SrpGenPublic() and SrpSetPrivate() with unit tests;
fixes k with left pad at g;
adds new error SRP_CALL_ORDER_E to force the functions to be called in the right order.
api tests refectory with ifdef and prototypes cleaning + use of asserts. Now, if the tests fails, it will point the line error and expected behavior like this:
ERROR - tests/api.c line 715 failed with:
test: server_args.return_code is true
result: server_args.return_code => FALSE
