S3 register handling for R (< 3.6)

Use of namespaced `S3method(...)` entries in `NAMESPACE` is not allowed in older versions of R. This was added to interoperate with `lucidate`, adding entries such as:

```
S3method(lucidate::year,partial_time)
```

This isn't strictly required, but makes iteration much easier. For now, an R version minimum has been introduced in `Depends`, but this could be loosened if this was rewritten to not require this feature.

To do so, we'd need to remove any of these entries in `NAMESPACE` and incorporate [`vctrs`-style S3 registration](https://vctrs.r-lib.org/reference/s3_register.html). Since this package is tightly coupled to `vctrs` already, there is no need to copy the code - it can be used directly from the `vctrs` namespace.

Start with the namespaced S3method(...) entries in NAMESPACE, the R version minimum in Depends, and the vctrs s3_register documentation linked in the issue. Confirm how the package currently registers these methods and whether vctrs can replace the entries; done means older R versions no longer require the current NAMESPACE feature.

Written by the indexing model from the issue text.