Make sqlite3ext optional

[Sytten]

• Upgraded bindgen
• Fixed your u32 vs i32 issue
• Made the sys crate optional (enabled by default)

This is technically a breaking change since we changed the return value of some functions.

I decided to make the new dynamic feature mutually exclusive with static to avoid duplicate definitions, but unsure if this is the best idea (you will probably want to disable dynamic if you enable static to avoid the bindgen). There is also the annoyance that each crate that depends on this one must also offer the two features (since features are additive). Unsure how best to best with this, I would need to check what does do.

[Sytten]

@asg017 Any chance to get that merged?