Make Migrations::new const #992

Job	Run time
Clippy (main package)	7s
Check	15s
Clippy (all code)	6s
Code coverage	1m 29s
Documentation	17s
Rustfmt	10s
Check with minimal Rust version	45s
Mutation tests (0)	1m 5s
Test all features	49s
Mutation tests (1)	1m 5s
Test feature alpha-async-tokio-rusqlite	36s
Test examples	36s
Mutation tests (2)	1m 4s
Mutation tests (3)	1m 8s
Test feature from-directory	37s
Test no features	37s
	10m 46s

Provide feedback