Previously get_arg_values was returning a vector of uint64_t values
that could be passed directly to printf(3). For string values
get_arg_values was returning a pointer to a char*. For some cases it
was attempting to handle freeing the char* memory via a stack
allocated std::vector. Unfortunately, this was stack allocated in
get_arg_values so the char* data would get freed before it was used in
the subsequent call to printf().

In other cases get_arg_values was not freeing char* values and was
leaking memory (probe, stack, and ustack).

get_arg_values() now returns a vector of objects of type IPrintable
instead of uint64_t values. Each object has a method .value() that
returns the uint64_t value usable by printf(). For strings this allows
us to keep around the original std::string until after we've called
printf(), so we don't need to strdup() anymore.

Fixes #194