Move back to erased_serde v0.3

The upgrade from erased_serde v0.3 to v0.4 in v2.8.0 was a breaking change, as it affected the public API. In the future, we may find a way to upgrade in a backwards-compatible manner. This should not affect normal users of the API but could break drains supporting nested-values.

I (@Techcable) apologize for this breakage and have yanked the 2.8.0 release due to this issue. I will try to setup cargo-semver-checks in the CI to avoid this in the future.

Fixed

• Restore compatibility with erased-serde version 0.3.
  • Compatibility with erased-serde version 0.4 may be provided again in a
    future version, but will be done so in a backwards compatible way.
  • Thanks to @demurgos for recognizing and fixing this in PR #359
• Ensure compatibility with -Zminimal-versions flag by specifying minimal dependency versions (fixes #357)