Choose appropriate name for the `edgedb-protocol::model` module

Previous discussion: https://github.com/edgedb/edgedb-rust/pull/44 and https://github.com/edgedb/edgedb-rust/issues/38

Options so far:
1. `data_types` is what is used in Python (or rather `datatypes`), but can be perceived to long for referencing too often
2. Keep it `model` (the name was was not discussed beforehand so should not be considered status quo)
3. `data` is shorter for data types
4. `scalars` -- but this might stop using it for more complex types if we need in future
5. Make it a separate crate, e.g. `edgedb-types`

Personally, I still like `model`, but `data` is also fine.

Read the previous discussion in pull request #44 and issue #38, then review the listed naming options for the edgedb-protocol::model module. The work is done when the project has a settled module or crate name and the relevant references consistently use that decision.

Written by the indexing model from the issue text.