Promotes `head`, improves `inspect`

[zslayton]

• Promotes head out of the beta namespace
• Improvements to inspect:
  • Confirms its input is a supported encoding
  • Simplifies (and fixes a bug in) the symbol ID assignment logic
  • Removes workaround for body_span now-fixed bug in ion_rs
  • Makes --no-auto-decompress a supported-by-default flag for all subcommands.
• Hides the dump alias for cat from --help's list of subcommands.
• Bumps ion_rs dependency to v1.0.0-rc.6

---

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[zslayton]

The CI checks are failing because the ion_rs dependency is pointing to an old commit.