Merge branch 'ps/reftable-fixes-and-optims'

More fixes and optimizations to the reftable backend.

* ps/reftable-fixes-and-optims:
  reftable/merged: transfer ownership of records when iterating
  reftable/merged: really reuse buffers to compute record keys
  reftable/record: store "val2" hashes as static arrays
  reftable/record: store "val1" hashes as static arrays
  reftable/record: constify some parts of the interface
  reftable/writer: fix index corruption when writing multiple indices
  reftable/stack: do not auto-compact twice in `reftable_stack_add()`
  reftable/stack: do not overwrite errors when compacting

diff --cc reftable/merged.c
Simple merge

diff --cc reftable/merged_test.c
Simple merge

diff --cc reftable/readwrite_test.c
Simple merge

diff --cc reftable/stack_test.c
Simple merge