Implement Spanner Graph queries

[apstndb]

This PR implements #98 except DDLs.
I want to separate {CREATE | DROP} PROPERTY GRAPH DDL to another PR.

• GRAPH_TABLE operator
• Graph subqueries as GoogleSQL expression
• Graph functions/predicates as GoogleSQL expression
• GQL statements
  • FILTER statement
  • FOR statement
  • LET statement
  • LIMIT statement
  • MATCH statement
  • NEXT statement
  • OFFSET statement
  • ORDER BY statement
    • collation
  • RETURN statement
  • SKIP statement
  • WITH statement
  • Set operation
  • Graph hints
• Path Patterns
• Misc
  • Add tests
  • Support in ./tools/parse
  • Implement all Pos() and End()
  • Clean up
  • Add helper functions (requires Go 1.20 because sqlOpt requires comparable with interface)
    • Bump Go version to 1.20
  • Add godoc