Pass free variables as args to pthread create

[mukul-rathi]

e.g.

async{
   let w = x.f + y.g
}

Here x and y are free variables in async - so need to be passed in as arg:

struct {
   x, 
   y
}

to pthread_create

[mukul-rathi]

Resolved by 86deeb182e9e4bc0a7deb41885364ffbbca3d73e, a908e0c5ef8d07cebc2a153b55ef501c2bad7fb8