pipes to `. <- ...`

[moodymudskipper]

The base pipe creates nested calls that are hard to debug, the magrittr pipe is not obvious to debug either.
The base pipe looks even worse if we don't have the srcref, which is the case for most packages.

When preprocessing we can transform this :

res <-
  foo |>
  fun1(a) |>
  fun2(b, c =  _)

into this :

. <- foo
. <- fun1(., a)
. <- fun2(b, c= .)
res <- . 

or

`1` <- foo
`2` <- fun1(`1`, a)
`3` <- fun2(b, c= `2`)
res <- `3`