Add try_bind function

[erikschul]

cornucopia can panic when types are incorrect during row.get. This can cause a server to crash.
This can occur when a table schema changes, and in particular, if nullability changes aren't updated correctly in all queries. (#226)

Can you provide a function try_bind that uses try_get?

See this example: https://github.com/Dowwie/tokio-postgres-mapper