Refactor: Remove legacy pymultihash backward compatibility APIs

Description

The py-multihash repository currently retains backwards-compatible APIs and constructor patterns originally introduced to facilitate migration from the legacy pymultihash package. As these compatibility layers have served their purpose and are now obsolete, this issue proposes their complete removal to streamline the codebase and improve long-term maintainability.

Proposed Changes

• Remove Legacy Constructor: Remove the 2-argument constructor Multihash(func, digest) fallback from the Multihash object.
• Enforce Canonical Initialization: Revert Multihash to standard namedtuple behavior using strictly (code, name, length, digest).
• Refactor Internal Helpers: Update digest() and sum_stream() helper functions to explicitly compute and construct the Multihash object using the canonical (code, name, length, digest) signature.
• Test Suite Updates: Remove corresponding test cases that checked the legacy initialization style.

Start by locating the Multihash object and the digest() and sum_stream() helpers in the py-multihash repository, then find tests covering the legacy initialization style. Verify the canonical (code, name, length, digest) initialization path and update the affected tests. Done means the two-argument constructor is no longer supported and the test suite passes with the helper changes.