Implement Spanner Graph queries

[apstndb]

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

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