Go implementations for improved performance and capabilities

[verdverm]

Today, the implementations are in CUE and entry is at the top level. Go implementations would likely have better performance. Additionally, they may open up features like Pattern Constraints l: [string]: string and Closedness close(), #def, and ...

The idea would be to walk values and the AST as needed.

• count
• depth
• diff
• patch
• pick
• mask
• replace
• insert
• upsert
• transform
• validate

[verdverm]

v0.3.1 CLI is now using the Go implementations. They pass the existing tests. We need more tests to verify more capabilities and edge cases. More improvements are planned.