Assorted improvements

[redneb]

This pull request contains a bunch of assorted additions to the library. I tried to break this into multiple PRs but this resulted in a merge conflict hell. I think it is easier to merge as a single PR and you can still review the individual commits. If you prefer multiple PRs, I can do it.

Here's an overview of the commits:

• add function for retrieving the db handle from a custom function context This is useful when you want to write a custom function that itself performs some sql queries.
• _add low-level bindings to sqlite3_walhook This is useful when you want to manually control when checkpointing occurs in wal journal mode. Unfortunately, it is not possible to provide high-level bindings to this function in haskell without leaking a FunPtr. Thus I only provide the low-level bindings and let the user manage the allocation/deallocation of the FunPtr.
• add support for activating/deactivating the shared cache mode This is useful when you have multiple connections to the same database in the same process.
• add support for zeroblobs This is needed for the incremental blob I/O.
• add support for incremental blob I/O
• _add bindings for sqlite3errcode This is needed for the online backup api.
• add support for the online backup api

[nurpax]

Looks very good in general, thanks for your contribution! No need to split into separate PRs.

You have really good information in your PR description some of which is not available in commit messages or in code comments. E.g.

This is useful when you want to manually control when checkpointing occurs in wal journal mode. Unfortunately, it is not possible to provide high-level bindings to this function in haskell without leaking a FunPtr. Thus I only provide the low-level bindings and let the user manage the allocation/deallocation of the FunPtr.

I think this could go into the function doc comment too? Similarly, it wouldn't hurt for this to be in commit messages. Commit message is what many people will read when doing code archaelogy later. It's not easy to go back to PR descriptions from individual commits.

I will read this in more detail later today/this week, but thus far I don't have any complaints. I'm glad you also added some tests for these new features!

[redneb]

I can certainly add that information to the commit messages but that would require rebasing and sending a new PR. So I will hold this off just in case you have more comments.

[IreneKnapp]

Oh, hey! The online backup API! Thank you! That's been a highly-wanted for quite some time. (In fact, there's an open issue asking for it.)

[nurpax]

Great work, I reviewed all the changes now and I don't have any complaints. Thank you very much for this contribution.

If you want to follow up with docs changes, for example for the wal_hook comment, feel free. But I think this is releasable as is, and I will soon upload this on hackage.