postgres/v0.2.0

Added

• MapError classifies SQLSTATE class-23 constraint violations into the base package's
  sentinels — unique (23505), foreign-key (23503), check (23514), not-null (23502) — through
  a ConstraintError carrying the constraint name pgx exposes structurally; every other
  error, sql.ErrNoRows included, passes through unchanged.
• The dialect implements ast.ReturningRenderer, the declared-native returning capability,
  emitting the postgres RETURNING form for the write statements.

Changed

• The go-database pin moves to v0.3.0, the release carrying the ast/operation/exec
  layer ontology this provider's capabilities plug into.