feat: adding sync support

[SilentVoid13]

I really enjoy welds' simple ergonomics as an ORM. However one thing I miss is sync support, as I prefer it over async in most cases. Making a whole program async just to communicate with the database isn't ideal.

This PR adds sync support via maybe_async.
This has multiple advantages:

• No code duplication, which means no added maintenance burden
• Easy to understand/use (a simple proc-macro)

Here is a summary of the changes I made to support sync:

• welds: welds' core was already more or less async-free, it only required it to communicate with welds-connection. Adding a bunch of #[maybe_async::maybe_async] was enough to convert it.
• welds-macros: changes were minimal as well, it only required a few tweaks to only emit async fn/ .await in the async case
• welds-connections: changes are more consequent as I added a new synchronous client to use with the new sync API. It's an sqlite client using rusqlite

New features added:

• __sync: internal feature used to enable/disable sync code compilation
• sqlite-sync: enables the sync sqlite client
• full-sync: same as full but for sync features

A couple of things to note:

• sync/async code is mutually exclusive, you can't compile both sync and async code at the same time. Allowing such a use case would require a much more consequent refactor
• I had to pin the exact rusqlite version because the libsqlite3-sys version was conflicting with sqlx. Maybe there's a more elegant solution

Although this PR might seem big, it's mostly just slapping a bunch of #[maybe_async::maybe_async] to enable both async and sync code. Hope you'll consider merging it!

[lex148]

This is awesome, yeah I want to merge it!
I am going to pull it into it's own branch first so I can add a bunch of tests and give it the works before it hits main.

Thanks for all the hard work!

[SilentVoid13]

Glad to hear that! Happy to help if you need or have any questions to improve the PR. Thank you for this great lib

[lex148]

Hey @SilentVoid13. thought you might like to know all the tests are passing and this has made it to main.

[SilentVoid13]

that's awesome, thank you for the speed!