Bump imhamt from a12e68d to f6bcb7f

Bumps [imhamt](https://github.com/input-output-hk/chain-libs) from `a12e68d` to `f6bcb7f`.
- [Release notes](https://github.com/input-output-hk/chain-libs/releases)
- [Commits](input-output-hk/chain-libs@a12e68d...f6bcb7f)

---
updated-dependencies:
- dependency-name: imhamt
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>