Replace mysql/postgres intersection interfaces with mysql type aliases
per go-jet/jet#555 and generalize Apply/Expr signatures.
Co-authored-by: Cursor <cursoragent@cursor.com>
Reorganize monolithic dbx.go and utility.go into focused files by
concern, add package doc.go, and gitignore coverage.out.
Co-authored-by: Cursor <cursoragent@cursor.com>