Revisit `vec_proxy` / `vec_restore` implementation

[sebkopf]

Currently chemunits is defining vec_proxy and vec_restore S3 methods in addition to ptype2 cast and

is the proxy stuff necessary? seems to be because of the way it is implemented in units but maybe if the vec_ptype2 and vec_cast methods from units are fully export it would work without?

https://github.com/r-quantities/units/blob/68f976ff715b4635c2de3241db4d65d1a626db99/R/tidyverse.R#L27