[Request] Add qs backend

[artemklevtsov]

Hi,

It would be nice to add qs as drop in replacement for the RDS.

[torfason]

And perhaps even more pertinent to add option to use qs::qserialize() as an option for driver_dbi (and perhaps other). A very quick and dirty replacement got some example data (character vectors) in SQLite down by almost 90%, from 4.4MB down to 550Kb. The time to write it was very similar, qs may be a little bit slower, but no more than 5-10%, and sometimes it comes out faster.

What I changed was just two functions:

serialize_to_raw <- function(x, ascii, xdr) {
  #serialize(x, NULL, ascii = ascii, xdr = xdr, version = 2L)
  qs::qserialize(x)
}

unserialize_safe <- function(x) {
  if (is.character(x)) {
    #unserialize(charToRaw(x))
    qs::qdeserialize(charToRaw(x))
  } else if (is.raw(x)) {
    #unserialize(x)
    qs::qdeserialize(x)
  } else {
    stop("Invalid input")
  }
}

I'm sure more would have to be changed for an actual implementation, and a decision would need to be made on how to offer configuration, as well as whether to save the serialization format as metadata in the underlying store in order to allow it to be read without knowing the serialization method beforehand. But for a storage engine, 90% space savings are pretty significant.