S3 register handling for R (< 3.6)

[dgkf]

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. 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.