remove NodePtr hack and fix-up benchmark-clvm-cost

[arvidn]

This fixes benchmark-clvm-cost to not need a magic NodePtr value for a variable placeholder. Instead, it uses Option<NodePtr> as it should have.

This let us remove the NodePtr::hack() constructor.

[coveralls-official[bot]]

Pull Request Test Coverage Report for Build 7831421212

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.09%) to 94.242%

---

Totals
Change from base Build 7792995744:	0.09%
Covered Lines:	5401
Relevant Lines:	5731

---

💛 - Coveralls