[CHIA-3002] optimize edge cases of concat

[arvidn]

In the case where a single argument is passed to concat, we can return the same node. In the case where 0 arguments are passed to concat, we can return NIL. Any NIL arguments passed to concat can be omitted, funnelling those cases towards one of the previous edge cases.

The current implementation copies the bytes on the heap of the node into a new allocation, returning a new node pointing to that heap region. This wastes memory in the case of a single node.

However, we impose limits on node count and heap size. In order to make these optimizations maintain the same behavior , we need to account for optimized away heap space. This patch calls it ghost_heap and harmonizes the naming of the similar counters to ghost_atoms and ghost_pairs. Those also track atoms and pairs that count against the limit, but were optimized away.

[coveralls-official]

Pull Request Test Coverage Report for Build 15463536475

Details

• 68 of 68 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 94.329%

---

Totals
Change from base Build 15209766231:	0.03%
Covered Lines:	6920
Relevant Lines:	7336

---

💛 - Coveralls

[richardkiss]

Seems reasonable. I dunno if it's worth the effort, although the effort is spent now. :)