Convert async to ALTREP

[yutannihilation]

Will it be useful?

[yutannihilation]

PoC: https://github.com/yutannihilation/savvy-async-poc

Random thoughts.

• Apparently, R API cannot be called in parallel.
  • Probably, the SEXP should be allocated beforehand and write the the underlying array?
  • But, it might not be always the case that the length is known.
  • Can I prohibit the use of APIs?

[yutannihilation]

• Probably, the SEXP should be allocated beforehand and write the the underlying array?

After some thinking, with the current implementation, there are almost no problem because such methods as etl() and copy_data() returns the primitive values and it's converted after the future returns the values.

One problem is that AltList currently requires the return type of Sexp. This should probably impl TryInto<Sexp> or something.

https://yutannihilation.github.io/savvy/reference/savvy/altrep/trait.AltList.html