Use srcref with language objects

[moodymudskipper]

To be able to keep comments, skipped lines, indention and other spaces in constructed output. As we do with functions.

See #215 (comment)

The srcref of language objects seems to work a bit differently.

x <- quote({
  a$b # some comment
})

attributes(x)
#> $srcref
#> $srcref[[1]]
#> {
#> 
#> $srcref[[2]]
#> a$b
#> 
#> 
#> $srcfile
#> <text> 
#> 
#> $wholeSrcref
#> x <- quote({
#>   a$b # some comment
#> }

cc @krlmlr