Release v0.12.2

What's Changed

• Remove derivative in favour of manual Debug impl by @macladson in #61
• Use div_ceil in tree_hash impl by @macladson in #63
• Add new method repeat_full to VariableList by @macladson in #62
• Release 0.12.2 by @macladson in #64

Full Changelog: v0.12.1...v0.12.2