Function migrate 

pub async fn migrate(
    pool: &PgPool,
    dir: &Path,
) -> Result<Vec<Migration>, MigrateError>

Ensure the ledger table exists, then apply every pending migration in dir (lex-sorted by name) to pool. Already-applied migrations are skipped.

Each migration runs in its own transaction unless its atomic field is false (e.g. for CREATE INDEX CONCURRENTLY). On failure within an atomic migration the file’s changes roll back cleanly; prior files stay applied (their commits already happened), so re-running migrate after fixing the offender will pick up where it left off.

Returns the migrations that were newly applied (could be empty).

Errors

Returns MigrateError::Io/MigrateError::Json/MigrateError::Validation for file problems, MigrateError::Driver for SQL failures.