cgraph: implement a generic stack abstraction

cgraph: implement a generic stack abstraction

Similar to prior abstractions like `bitarray_t`, this is implemented header-only
so as to be usable throughout the Graphviz tree, even by code that is not
linking against cgraph.

Given this implementation is header-only, it is natural to wonder why the type
needs a `gv_` prefix. The answer is that one of the macOS system headers flouts
the rule of `__` prefixing symbols that are part of the implementation and
defines a typedef of `__darwin_sigaltstack` under the name `stack_t`. Hence this
name is not usable by us.

Gitlab: #1793, #2222