Proposal: migrate spansql with memefish

[apstndb]

WHAT

I think it is better to replace spansql with memefish in wrench.

WHY

Related discussion: #83

It seems that wrench users sometimes contribute spansql to fix their issues, but it is considered a loss of community resources. https://github.com/googleapis/google-cloud-go/commits/main/spanner/spansql

spansql has many incompatibilities, and it seems no chance to become a real SQL parser. It don't support basic SQL feature like subqueries.

result of memefish test cases in spansql: https://github.com/apstndb/spansql-testsuite/blob/abc0bd3c3ee11f53d81c6f83f0494b56fba9ee20/output/result.csv