Merge 'Replace scalar function declaration in extension API with proc macro' from Preston Thorpe

Per @penberg's suggestion on Discord, the `declare_scalar_functions!`
declarative macro has been replaced with a procedural one, allowing for
a much nicer API.
```rust
#[export_scalar]
#[args(0)]
fn uuid4_blob(_args: &[Value]) -> Value {
    let uuid = uuid::Uuid::new_v4();
    let bytes = uuid.as_bytes();
    Value::from_blob(bytes.to_vec())
}
```

Closes #687

Author: penberg