gc: replace inline stack implementation with generic one

gc: replace inline stack implementation with generic one

This code was using two abstractions, a block `blk_t` and stack `stk_t`, to
amortize the cost of allocations. We can remove the block abstraction and
rewrite the stack implementation to use the simpler generic stack while still
retaining these amortization benefits. Note that this refactoring also makes
initialization of the stack data structure unnecessary as a zeroed `gv_stack_t`
is also a valid empty stack.

The new code also deallocates the stack prior to exit, aiding tools like
Valgrind and Address Sanitizer.

This pattern of using both a hand-rolled block and hand-rolled stack appears in
numerous places in the Graphviz code base, of which this is just one instance.

Gitlab: #1793