devel/R-cran-listenv: Add new port
List environments are environments that have list-like properties.
For instance, the elements of a list environment are ordered and
can be accessed and iterated over using index subsetting,
e.g.
'x <- listenv(a = 1, b = 2);
for (i in seq_along(x)) x[[i]] <- x[[i]] ^ 2; y <- as.list(x)'.