Support crate for SQLx's proc macros.
### Note: Semver Exempt API
The API of this crate is not meant for general use and does *not* follow Semantic Versioning.
The only crate that follows Semantic Versioning in the project is the `sqlx` crate itself.
If you are building a custom SQLx driver, you should pin an exact version of this and
`sqlx-core` to avoid breakages:
```toml
sqlx-core = "=0.6.2"
sqlx-macros-core = "=0.6.2"
```
And then make releases in lockstep with `sqlx-core`. We recommend all driver crates, in-tree
or otherwise, use the same version numbers as `sqlx-core` to avoid confusion.