benches: Added bench_rbyd, bench_wt, and bench_helpers

These were copied from external benchmarks, and tweaked/simplified a
bit based on gained experience.

I mostly just wanted something to test the bench runner/scripts, with
bench_rbyd showcasing a low-level litmus benchmark, and bench_wt
showcasing a high-level throughput benchmark.

Though bench_wt has proven to be a _very_ versatile benchmark, and will
likely be the first stop for getting an understanding of high-level
performance implications.

---

Also added bench_helpers.h/c, which includes a couple helper functions:

- bench_helpers_warmup - Warm up the filesystem by writing a 1 block
  file 2*block_count times. This is meant to exhaust any preerased
  state, post-format lookahead buffers, etc.

- bench_helpers_usage - Find a tight bound on disk usage. This allocates
  a bitmap to find the tight bound, unlike lfs3_fs_usage, which is
  best-effort. However the bitmap is hidden behind BENCH_HEAP_PAUSE to
  prevent messing with parallel heap measurements.

benches/bench_helpers.h

@@ -0,0 +1,22 @@
+/*
+ * Some extra bench helpers
+ *
+ */
+#ifndef BENCH_HELPERS_H
+#define BENCH_HELPERS_H
+
+#include "runners/bench_runner.h"
+
+
+// warm up the filesystem
+//
+// this writes a 1 block file 2*block_count times to get it into a good
+// state for benchmarking
+int bench_helpers_warmup(lfs3_t *lfs3);
+
+
+// find tight disk usage
+uintmax_t bench_helpers_usage(lfs3_t *lfs3);
+
+
+#endif