copy does not overallocate nested lists of data.tables

[d-sci]

Similar to #1476 , if I copy a list of lists (of lists ...) of data.tables I will get the notorious "Invalid .internal.selfref detected" warning when later modifying the data.table elements by reference. While this has since been fixed for lists one layer deep, for people who use data.table as their main form of data representation, it is not so rare to have nested lists of data.tables, and an occasional need to take a deep copy.

library(data.table)
x = list(a = data.table("A"), b = list(bb = data.table("B")))
y = copy(x)

# All good one layer deep
y$a[, new := "new"]
y$a
#>    V1 new
#> 1:  A new

# Warning on second layer
record <- y$b$bb[, new := "new"]
#> Warning in `[.data.table`(y$b$bb, , `:=`(new, "new")): Invalid .internal.selfref
#> detected and fixed by taking a (shallow) copy of the data.table so that :=
#> can add this new column by reference. At an earlier point, this data.table
#> has been copied by R (or was created manually using structure() or similar).
#> Avoid names<- and attr<- which in R currently (and oddly) may copy the whole
#> data.table. Use set* syntax instead to avoid copying: ?set, ?setnames and ?
#> setattr. If this message doesn't help, please report your use case to the
#> data.table issue tracker so the root cause can be fixed or this message
#> improved.
# And although it did something ...
record[]
#>    V1 new
#> 1:  B new
# ... it didn't persist
y$b$bb
#>    V1
#> 1:  B

sessionInfo()
#> R version 3.5.2 (2018-12-20)
#> Platform: x86_64-apple-darwin15.6.0 (64-bit)
#> Running under: macOS Mojave 10.14.6
#> 
#> Matrix products: default
#> BLAS: /Library/Frameworks/R.framework/Versions/3.5/Resources/lib/libRblas.0.dylib
#> LAPACK: /Library/Frameworks/R.framework/Versions/3.5/Resources/lib/libRlapack.dylib
#> 
#> locale:
#> [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
#> 
#> attached base packages:
#> [1] stats     graphics  grDevices utils     datasets  methods   base     
#> 
#> other attached packages:
#> [1] data.table_1.12.8
#> 
#> loaded via a namespace (and not attached):
#>  [1] compiler_3.5.2  magrittr_1.5    tools_3.5.2     htmltools_0.4.0
#>  [5] yaml_2.2.0      Rcpp_1.0.3      stringi_1.4.3   rmarkdown_1.16 
#>  [9] highr_0.8       knitr_1.26      stringr_1.4.0   xfun_0.11      
#> [13] digest_0.6.22   rlang_0.4.2     evaluate_0.14

Proposed solution to follow.