Fix int4 decoding/encoding

[Lagovas]

Found that we incorrectly decode signed int4 values by checking value of last byte instead of last bit that represents sign of integer. Fixed and added tests that verify it. P.S. additionally fixed some golint issues that blocks acra-ee

Checklist

• [x] Change is covered by automated tests
• [x] The coding guidelines are followed
• [x] Public API has proper documentation in the Acra documentation site or has PR on documentation repository with new changes
• [ ] CHANGELOG.md is updated (in case of notable or breaking changes)
• [x] CHANGELOG_DEV.md is updated
• [x] Benchmark results are attached (if applicable)
• x are up-to-date (in case of API changes)