v0.6.0-rc.1

Thank you to @mpvl, and @mvdan for contributing to this release!

Changelog

• internal/core/adt: lets of comprehensions are multi lets by @mpvl in 43b4674
• internal/core/adt: always mark let in comprehension as "complete" by @mpvl in 5123737
• cue/ast: clarify what is a valid identifier by @mvdan in c4ced5e
• internal/core/adt: find late-stage conflict errors by @mpvl in 0c48837
• internal/core/adt: add tests to fix 2481 by @mpvl in d3444e3