Adds a --sqlar option that defaults insert-files to the name/mode/mtime/sz/data
schema used by SQLite's own sqlar archive format, plus a content_sqlar column
type that zlib-compresses content the same way sqlar_compress() does (only
when compression actually shrinks the data). Reuses the existing -c coldef
parsing added for fixed-literal metadata columns, no changes needed there.
Existing BLOB/TEXT content/content_text imports are untouched.
Closes#141