Provide documentation for procedural macros in scylla-udf-macros, not scylla-udf

[piodul]

Currently, the project has two crates: scylla-udf and scylla-udf-macros. The latter defines procedural macros that use some items from the first one, and also the former re-exports the macros defined in the latter. Due to that, we thought that it's impossible to provide doctests in the scylla-udf-macros crate: we would have to import scylla-udf in scylla-udf-macros, which would cause an import cycle.

However, it appears that it's possible for scylla-udf-macros to have a dev-dependency on scylla-udf - meaning that the test binary for scylla-udf-macros depends on scylla-udf, but not the crate itself. The procedural macros are not documented in scylla-udf-macros, but we could easily fix that by introducing the dev-dependency and moving the docstrings to the crate where the macros are defined.