Naive search internals (core only)

[mitchellh]

Towards #189

This PR implements a simple, single-threaded, single-direction Booyer-Moore-Horspool search on top of PageList.

The code in this PR is not utilized in builds yet. It is only part of unit tests. I want to open a PR incrementally so we don't have one mega PR for search. I want to be very clear that this does not finish #189, there's many more steps before that.

High-level technical overview: we maintain a circular buffer of UTF-8 encoded terminal page contents that is just large enough to fit the larger of a single page or the search term. As we search, the circular buffer is pruned and additional terminal pages are added to search. This minimizes our memory usage to the minimum necessary for this approach. In practice, the overhead is very small for realistic search terms!

Some limitations that need to be resolved in future work:

• Single-direction: We only search oldest-to-newest. We'll want to search newest-to-oldest by default. It shouldn't be hard to implement directionality but I wanted to wait until I needed it.
• Single-threaded: Search only works as long as the PageList is not being actively modified. That's not going to work for a real final user-facing solution.

[mitchellh]

Going to merge this. This isn't in use at all yet so there's no potential harm here in a merge. I'll still be notified and read any feedback if anyone has any. Then I'll plan follow ups to keep working on this...