Nested pa_* support

[albert-ying]

Currently the nested process gives error (at least in linux)

pa_walk(c(1:10), ~{
  pa_walk(c(1:10), ~{
    print(.x)
  }, cores = 2)
}, cores = 2)

Error in { : 
  task 1 failed - "the ... list contains fewer than 2 elements"

Interestingly, the error message changed sometimes (usually when used more cores)

Error in { : 
  task 1 failed - "creation of server socket failed: port 11494 cannot be opened"

[moosa-r]

Thank you very much for reporting this issue. It is most probably due to scoping and the environment that the .f expression is being evaluated. I will look into it.