Allow mutable ALTREP

[Hiroaki Yutani]

At least there can be three strategies on modification

1. immutable
   1. always duplicate and modify the new one
2. mutable
   1. modify the materialized version of SEXP in data2 (or somewhere linked to data2)
   2. modify the underlying *mut T, if available

I haven't seen an implementation that chose 2-ii, but it might be the case with Rust.

[Hiroaki Yutani]

It seems this is not possible. Closing.

#223 (comment)