perf: use setDT() inside map_dtc() call

[m-muecke]

closes: #78, see the following:

input	old (do.call)	new (setDT)
runif(1e6, max = x)	30 ms	20 ms
data.table(x = runif(1e6, ...))	23 ms	19 ms

bench::mark medians, iterations sized per case for stability:

case	main (do.call)	new (setDT)	speedup	main mem	new mem
vectors (10 × 1000 rows)	157 µs	89.3 µs	1.76x	178 KB	95 KB
small dt (10 × 100 rows)	624 µs	344 µs	1.81x	209 KB	194 KB
medium dt (10 × 10 000 rows)	1.24 ms	889 µs	1.40x	2.47 MB	1.7 MB
large dt (3 × 1 000 000 rows)	21.5 ms	18.2 ms	1.18x	68.7 MB	45.8 MB
wide dt (5 × 100 cols × 1000 rows)	4.84 ms	2.21 ms	2.19x	228 KB	207 KB
mixed atomic + data.table	83.4 µs	39.9 µs	2.09x	32 KB	16 KB
matrix (5 × 100 cols × 1000 rows)	7.72 ms	4.4 ms	1.75x	17.5 MB	13.5 MB