sumdb/storage: basic storage interface and tests

Copied from golang.org/x/exp/sumdb/internal/tkv and tkvtest.

For golang/go#31761.

Change-Id: Ib5a411c9b9641d1006a2e721043e8e16bf0f6bea
Reviewed-on: https://go-review.googlesource.com/c/mod/+/176465
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
4 files changed
tree: a9406cb788c8945be0e9a58406c0f8ad0ae5bd4a
  1. LICENSE
  2. PATENTS
  3. README
  4. codereview.cfg
  5. go.mod
  6. go.sum
  7. module/
  8. semver/
  9. sumdb/