Various buffer optimizations (#1211)

This PR is a set of small cleanups to reduce `memcpy` and `memset` calls in
`Buffer` wrangling.

- `Buffer::eat` just swaps pointers if it's a complete (fully owned) overwrite,
  instead of using `memcpy`
- `Buffer::eat` no longer sets the size of the eaten buffer to 0, to avoid
  `memset` when scaling it up later
- Added a new `Buffer:reset_owned`, which resizes and clears the `owned` flags
  but **does not** clear buffer data (for the case where we're about to
  overwrite buffer data anyways)
    - Use this new function in `Volume::read`
- In `Volume::read`, read directly into the provided buffer if there's only one
  subvolume (instead of reading into a scratch buffer then copying)
    - The scratch buffer is no longer always allocated; it's resized if needed