sled 0.14.1

a flash-sympathetic persistent lock-free B+ tree, pagecache, and log
Documentation
Now that some property and sanitization tests exist, it's safe to open this up for wider contribution!

Specifically, there are a few areas that I would love external help with:

* performance tuning: this should theoretically be able to outperform rocksdb on reads,
  and innodb on writes! let's get there! I usually run this to generate and view flamegraphs:
  ```
  cargo build --bin=stress --features=stress,zstd; hack/flamerun.sh ./target/debug/stress --duration 5; firefox ./flamegraph.svg
  ```
* a better C API: the current one is pretty unfriendly
* more testing: ALICE for testing crash safety
* better docs: whatever you find confusing!
* the deep end: prototyping a compiler plugin that allows specifiable blocks of code to be
instrumented with pauses that cause interesting interleavings to be exhausted. I don't think
this can be done with just a macro yet...

General considerations:

* all PR's block on failing tests
* all PR's block on breaking API changes (with the sole exception of the emerging C API)