add halfmd5 -m 5100

[cyclone-github]

Note

Introduces a new hashing mode and refreshes help/docs for the v1.2.2 release.

• Adds halfmd5 (-m 5100) implementation in hashgen.go
• Updates -help output: documents -help/-version, lists halfmd5, and tidies mode listing (keccak grouped after SHA3)
• Bumps version to v1.2.2 and updates CHANGELOG.md
• Refreshes README.md examples/perf note and adds halfmd5 to the functions table

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