light setDT

[jangorecki]

Provides more lightweight interface for setDT. Use cases are rather uncommon, of using setDT in big loops, which is described in #4476.
It may be useful for our internal usage of setDT where its input is already produced by our code and not provided by user.
There was also request for interface to alloccol on C level, the C function here is well addressing that, #4439.

library(data.table)
df1 = data.frame(x=1:4, y=1:2)
df2 = copy(df1)
microbenchmark::microbenchmark(times=1000,
  normal = setDT(df1)[, sum(x), y],
  simple = setDT(df2, asis=TRUE, return=TRUE)[, sum(x), y]
)
#Unit: microseconds
#   expr     min       lq     mean   median       uq      max neval
# normal 654.000 676.3505 730.4088 707.9065 725.1735 9311.567  1000
# simple 571.862 594.5105 652.3667 626.2590 642.4785 8279.154  1000

microbenchmark::microbenchmark(times=1,
  normal = for (i in 1:1e3) setDT(df1),
  simple = for (i in 1:1e3) setDT(df2, asis=TRUE, return=TRUE)
)
#Unit: milliseconds
#   expr       min        lq      mean    median        uq       max neval
# normal 77.017866 77.017866 77.017866 77.017866 77.017866 77.017866     1
# simple  8.291629  8.291629  8.291629  8.291629  8.291629  8.291629     1

No tests yet. Setting WIP for now.
Will add tests and C exports for #4439 after initial round of feedback.