Growable ALTREP vectors

[aitap]

Ongoing work towards #6180. Depends on #6694. Detailed motivation in a pending blog post.

Good news:

* checking compiled code ... NOTE
File ‘data.table/libs/data_table.so’:
  Found non-API call to R: ‘LEVELS’ # <-- and that's it!

Bad news:

Test 2289.2 ran without errors but failed check that x equals y:
> x = dt[, b, a] 
   a b  [Key= Types=int,lis Classes=int,lis]
1: 1 1                                      
2: 2 2                                      
> y = dt 
   a b  [Key= Types=int,exp Classes=int,exp]
1: 1 1                                      
2: 2 2                                      
Datasets have different column modes. First 3: b(list!=expression)

Unfortunately, since there is no altexpression ALTREP class, best I was able to support EXPRSXP columns was by creating VECSXP lists for their contents.

How long do we have and does anyone have a plan B?