Make Migrations::new const #994

Job	Run time
Test no features	35s
Check	14s
Check with minimal Rust version	2m 19s
Rustfmt	6s
Documentation	14s
Test all features	42s
Test feature alpha-async-tokio-rusqlite	36s
Test examples	35s
Test feature from-directory	41s
Clippy (all code)	9s
Clippy (main package)	7s
Code coverage	1m 29s
Mutation tests (0)	1m 4s
Mutation tests (1)	1m 8s
Mutation tests (2)	1m 8s
Mutation tests (3)	1m 5s
	12m 12s

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