Bug fixes in GROQ implementation

This release fixes smaller bugs for GROQ v2021-03-25.

• Fixes a bug where dereferencing inside sliced arrays (e.g. *[_type == "book"]{"authors": authors[0...2]{era->}}) would return incorrect result.
• Fixes a bug where scoping was incorrect while dereferencing arrays: *[_type == "book"]{"authors": authors[]->{..., "bookName": ^.name}}
• select now produces a query error when given invalid arguments. Examples of invalid arguments are select(title, title2) (multiple fallback values) and select(title, isGood => title2) (alternatives after the fallback)