Proposal: Implement statement separator without spansql to fix parse errors

[apstndb]

WHAT

There is a motivation to replace spansql with a more primitive implementation.
I want to discuss it.

WHY

Currently, wrench can't process statements which are not supported by spansql because of the spansql dependency.

spansql is used in ddlToStatements, dmlToStatements.
https://github.com/cloudspannerecosystem/wrench/blob/v1.4.1/pkg/spanner/migration.go#L144-L170

spansql is not actively maintained by Google and not 100% compatible with Cloud Spanner DDL/DML so the dependency on spansql makes it complex for wrench to support the new features of Cloud Spanner.

Reference

• Cloud Spanner Standard SQL lexical structure
  • https://cloud.google.com/spanner/docs/reference/standard-sql/lexical?hl=en
  • We should only think about comments and literals.
• spansql is introduced to support DDLs/DMLs with comments.
  • all: use spansql for parse DDL #54
  • Comment preserve is not yet implemented Preserve comments for schema.sql #15
• spanner-cli implements statement separator with comment strip
  • Query having ; in the string literal fails execution in the batch mode spanner-cli#33
  • Statement separator spanner-cli#34
    • I think wrench can reuse this implementation.

[apstndb]

It is resolved by #118