Pre-compute initial explode actions

[ebblake]

Description

During part two, we are computing a pairing between every two lines. Any line that had one or more depth 4 number pairs will cause the same set of explode actions to be taken for each of the other 99 lines it is paired with, before we get into the main effort of add() in processing split actions. Rather than repeat early work that will produce the same answer, we can instead pre-compute these initial explode calls during parse time.

On my input, adding a simple counter shows that this reduces the number of calls to explode() from 180k down to 131k. parse() and part1() runtimes are about the same, while part2() speeds up from 600us to 560us.

Type of change

• Performance improvement
• Bug fix
• Other

Checklist

• Pull request title and commit messages are clear and informative.
• Documentation has been updated if necessary.
• Code style matches the existing code. This one is somewhat subjective, but try to "fit in" by
  using the same naming conventions. Code should be portable, avoiding any
  architecture-specific intrinsics.
• Tests pass cargo test
• Code is formatted cargo fmt -- `find . -name "*.rs"`
• Code is linted cargo clippy --all-targets --all-features

Formatting and linting also can be executed by running just
(if installed) on the command line at the project root.

[ebblake]

Inspired by https://www.reddit.com/r/adventofcode/comments/1uzta0a/comment/p0umy44/

[ebblake]

It would also be an interesting experiment (one I still need to run) to see if there is any reliable relation between magnitude(reduce([left,0])) and the eventual magnitude(reduce([left,right])) winner. For example, if the winning magnitude always depends on one of the top-ten magnitude values out of the 100 inputs being on the left, we could avoid a full 100*99 add() calls in part 2 and instead focus on the ones most likely to produce a large value. But it is complicated by the fact that split() means the mapping is not straightforward: [[0,[0,[0,0]]],[[[9,0],0],0]] has magnitude 486, but if any explode resulted in the intermediate [[0,[0,[0,0]]],[[[10,0],0],0]] that reduces to [[0,[0,[0,5]]],[[[0,5],0],0]] with magnitude 300.