Add a test utility for preparing statements without executing them

[csamak]

The intent here is to sanity check queries against a connection/schema without dealing with input parameters. I am using this with hasql-th for fast tests to give some confidence that queries still make basic sense after schema migrations.

This may help with some benefits discussed in #28 without the compile-time downsides.

[nikita-volkov]

Sorry. I've missed your PR. Seems like a nice addition!

How do you feel about adapting it to the Session interface?

[csamak]

Thanks for taking a look!

IIRC I started with Session but stopped to avoid using the encoder. I didn't have a good way to go that route without requiring inputs.