Adds {cnd}

[jmbarbone]

resolves #259

[jmbarbone]

Not going to be concerned with these errors

❯ checking examples with --run-donttest ... ERROR
  Running examples in ‘mark-Ex.R’ failed
  The error most likely occurred in:
  
  > base::assign(".ptime", proc.time(), pos = "CheckExEnv")
  > ### Name: are_identical
  > ### Title: Identical extensions
  > ### Aliases: are_identical
  > 
  > ### ** Examples
  > 
  > x <- y <- z <- 1:5
  > y[2] <- 3L
  > z[5] <- NA_integer_
  > 
  > identical(x, y)        # compare entire vector
  [1] FALSE
  > are_identical(x, y)    # element-wise
  Error in dyn.load(file, DLLpath = DLLpath, ...) : 
    unable to load shared object '/home/runner/work/_temp/Library/rlang/libs/rlang.so':
    /home/runner/work/_temp/Library/rlang/libs/rlang.so: undefined symbol: SETLENGTH
  Calls: are_identical -> loadNamespace -> library.dynam -> dyn.load
  Execution halted

❯ checking tests ...
  See below...

── Test failures ───────────────────────────────────────────────── spelling ────

> if (requireNamespace("spelling", quietly = TRUE)) {
+   spelling::spell_check_test(
+     vignettes = TRUE,
+     error = FALSE,
+     skip_on_cran = TRUE
+   )
+ }
Error in dyn.load(file, DLLpath = DLLpath, ...) : 
  unable to load shared object '/home/runner/work/_temp/Library/rlang/libs/rlang.so':
  /home/runner/work/_temp/Library/rlang/libs/rlang.so: undefined symbol: SETLENGTH
Calls: <Anonymous> ... namespaceImport -> loadNamespace -> library.dynam -> dyn.load
Execution halted
Error: Error: R CMD check found ERRORs